[HTC Sensation] Catnap:
Right, I finally caught a soft reboot after running logcat to a file for just over 5 hours.

I/PowerManagerService(29427): Going to sleep by user request...
D/SurfaceFlinger( 119): Screen released, type=0 flinger=0x1a8d318
V/KeyguardHostView(29427): Keyguard widgets disabled by DPM
V/KeyguardHostView(29427): Keyguard secure camera disabled by DPM
V/TransportControlView(29427): Create TCV com.android.internal.policy.impl.keyguard.KeyguardTransportControlView{413c8d78 V.E..... ......I. 0,0-0,0 #1020309 android:id/keyguard_transport_control}
D/Sensors (29427): handle : 4 en: 0, v010.0 - Try to resolve lock issue
D/Sensors (29427): lock_status = 4
D/Sensors (29427): Andy: MPLSensor::enable: lock1
W/MPL-sup (29427): MLGetAKMComassStatus, gAKMCompassStatus = 0
D/Sensors (29427): mEnabled = 0x0
D/Sensors (29427): set_power_states++: enabled_sensors: 0 dmp_started: 1
D/Sensors (29427): Before if statment: cs:0 rs:0 en_ped:0 da_ped:0 en_g:0 da_g:0
D/Sensors (29427): Stopping DMP
I/ (29427): mpu3050_suspend: suspending sensors to 0000
I/ (29427): mpu3050_suspend: Will resume next to 0070
D/Sensors (29427): inside if(s_use_timerirq)
D/Sensors (29427): set_power_states--
D/Sensors (29427): Andy: MPLSensor::enable: unlock1
D/Sensors (29427): MPLSensor::enable--1
D/dalvikvm(29427): GC_FOR_ALLOC freed 811K, 23% free 11692K/15152K, paused 72ms, total 73ms
E/KeyguardHostView(29427): Error when trying to bind default AppWidget: java.lang.IllegalArgumentException: not a appwidget provider: ComponentInfo{/}
D/PhoneStatusBar(29592): disable: < EXPAND* icons alerts TICKER* system_info back home RECENT* clock search >
D/memalloc( 119): Out of PMEM. Dumping PMEM stats for debugging
D/memalloc( 119): ------------- PRINT PMEM STATS --------------
D/memalloc( 119): Node 0 -> Start Address : 0 Size 65280 Free info 0
D/memalloc( 119): Node 1 -> Start Address : 65280 Size 65280 Free info 0
D/memalloc( 119): Node 2 -> Start Address : 130560 Size 65280 Free info 0
D/memalloc( 119): Node 3 -> Start Address : 195840 Size 13056 Free info 1
D/memalloc( 119): Node 4 -> Start Address : 208896 Size 65280 Free info 0
D/memalloc( 119): Node 5 -> Start Address : 274176 Size 52224 Free info 1
D/memalloc( 119): Node 6 -> Start Address : 326400 Size 130560 Free info 0
D/memalloc( 119): Node 7 -> Start Address : 456960 Size 28288 Free info 1
D/memalloc( 119): Node 8 -> Start Address : 485248 Size 65280 Free info 0
D/memalloc( 119): Node 9 -> Start Address : 550528 Size 36992 Free info 1
D/memalloc( 119): Node 10 -> Start Address : 587520 Size 65280 Free info 0
D/memalloc( 119): Node 11 -> Start Address : 652800 Size 65280 Free info 0
D/memalloc( 119): Node 12 -> Start Address : 718080 Size 65280 Free info 0
D/memalloc( 119): Node 13 -> Start Address : 783360 Size 65280 Free info 0
D/memalloc( 119): Node 14 -> Start Address : 848640 Size 65280 Free info 0
D/memalloc( 119): Node 15 -> Start Address : 913920 Size 65280 Free info 0
D/memalloc( 119): Node 16 -> Start Address : 979200 Size 65280 Free info 0
D/memalloc( 119): Node 17 -> Start Address : 1044480 Size 4096 Free info 1
D/memalloc( 119): Total Allocated: Total Free:
D/memalloc( 119): ----------------------------------------------
E/memalloc( 119): /dev/pmem: No more pmem available
W/memalloc( 119): Falling back to ashmem
E/memalloc( 119): alloc mmap(fd=72, size=2088960, prot=3) failed (Try again)
E/gralloc ( 119): gralloc failed err=Try again
W/GraphicBufferAllocator( 119): alloc(540, 960, 1, 00000b03, ...) failed -11 (Try again)
E/SurfaceFlinger( 119): GraphicBufferAlloc::createGraphicBuffer(w=540, h=960) failed (Try again), handle=0x0
E/BufferQueue( 119): [Keyguard] dequeueBuffer: SurfaceComposer::createGraphicBuffer failed
W/Adreno200-EGLSUB(29427): <DequeueBuffer:420>: dequeue native buffer fail: Try again
W/Adreno200-EGL(29427): <qeglDrvAPI_eglMakeCurrent:2850>: EGL_BAD_ALLOC
E/libEGL (29427): eglMakeCurrent:593 error 3003 (EGL_BAD_ALLOC)
W/dalvikvm(29427): threadid=13: thread exiting with uncaught exception (group=0x40b74930)
E/AndroidRuntime(29427): *** FATAL EXCEPTION IN SYSTEM PROCESS: UI
E/AndroidRuntime(29427): java.lang.IllegalStateException: eglMakeCurrent failed EGL_BAD_ALLOC
E/AndroidRuntime(29427):	at android.view.HardwareRenderer$GlRenderer.createSurface(HardwareRenderer.java:1069)
E/AndroidRuntime(29427):	at android.view.HardwareRenderer$GlRenderer.createEglSurface(HardwareRenderer.java:961)
E/AndroidRuntime(29427):	at android.view.HardwareRenderer$GlRenderer.initialize(HardwareRenderer.java:787)
E/AndroidRuntime(29427):	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1502)
E/AndroidRuntime(29427):	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
E/AndroidRuntime(29427):	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
E/AndroidRuntime(29427):	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
E/AndroidRuntime(29427):	at android.view.Choreographer.doCallbacks(Choreographer.java:562)
E/AndroidRuntime(29427):	at android.view.Choreographer.doFrame(Choreographer.java:532)
E/AndroidRuntime(29427):	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
E/AndroidRuntime(29427):	at android.os.Handler.handleCallback(Handler.java:725)
E/AndroidRuntime(29427):	at android.os.Handler.dispatchMessage(Handler.java:92)
E/AndroidRuntime(29427):	at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(29427):	at android.os.HandlerThread.run(HandlerThread.java:60)
D/PowerManagerService-JNI(29427): Excessive delay in autosuspend_enable() while turning screen off: 371ms
I/Process (29427): Sending signal. PID: 29427 SIG: 9
I/ServiceManager( 113): service 'appwidget' died
I/ServiceManager( 113): service 'alarm' died
I/ServiceManager( 113): service 'entropy' died
I/ServiceManager( 113): service 'samplingprofiler' died
I/ServiceManager( 113): service 'hardware' died
I/ServiceManager( 113): service 'power' died
I/ServiceManager( 113): service 'assetredirection' died
I/ServiceManager( 113): service 'backup' died
I/ServiceManager( 113): service 'batteryinfo' died
I/ServiceManager( 113): service 'package' died
I/ServiceManager( 113): service 'activity' died
I/ServiceManager( 113): service 'meminfo' died
I/ServiceManager( 113): service 'battery' died
I/ServiceManager( 113): service 'cpuinfo' died
I/ServiceManager( 113): service 'gfxinfo' died
I/ServiceManager( 113): service 'profile' died
I/ServiceManager( 113): service 'diskstats' died
I/ServiceManager( 113): service 'input' died
I/ServiceManager( 113): service 'device_policy' died
I/ServiceManager( 113): service 'netpolicy' died
I/ServiceManager( 113): service 'netstats' died
I/ServiceManager( 113): service 'statusbar' died
I/ServiceManager( 113): service 'user' died
I/ServiceManager( 113): service 'scheduling_policy' died
I/ServiceManager( 113): service 'display' died
I/ServiceManager( 113): service 'content' died
I/ServiceManager( 113): service 'telephony.registry' died
I/ServiceManager( 113): service 'dbinfo' died
I/ServiceManager( 113): service 'account' died
I/ServiceManager( 113): service 'wifi' died
I/ServiceManager( 113): service 'vibrator' died
I/ServiceManager( 113): service 'sensorservice' died
I/ServiceManager( 113): service 'window' died
I/ServiceManager( 113): service 'usagestats' died
I/ServiceManager( 113): service 'uimode' died
I/ServiceManager( 113): service 'permission' died
I/ServiceManager( 113): service 'lock_settings' died
I/ServiceManager( 113): service 'clipboard' died
I/ServiceManager( 113): service 'input_method' died
I/ServiceManager( 113): service 'accessibility' died
I/ServiceManager( 113): service 'network_management' died
I/ServiceManager( 113): service 'mount' died
I/ServiceManager( 113): service 'bluetooth_manager' died
I/ServiceManager( 113): service 'textservices' died
I/ServiceManager( 113): service 'wifip2p' died
I/ServiceManager( 113): service 'commontime_management' died
I/ServiceManager( 113): service 'serial' died
I/ServiceManager( 113): service 'connectivity' died
I/ServiceManager( 113): service 'servicediscovery' died
I/ServiceManager( 113): service 'throttle' died
I/ServiceManager( 113): service 'fm_receiver' died
I/ServiceManager( 113): service 'fm_transmitter' died
I/ServiceManager( 113): service 'updatelock' died
I/ServiceManager( 113): service 'dreams' died
I/ServiceManager( 113): service 'notification' died
I/ServiceManager( 113): service 'audio' died
I/ServiceManager( 113): service 'usb' died
I/ServiceManager( 113): service 'devicestoragemonitor' died
I/ServiceManager( 113): service 'location' died
I/ServiceManager( 113): service 'country_detector' died
I/ServiceManager( 113): service 'search' died
I/ServiceManager( 113): service 'dropbox' died
I/ServiceManager( 113): service 'wallpaper' died
W/Sensors (29774): sensorservice died [0x2025300]
W/Sensors (29740): sensorservice died [0x200bd48]
W/Sensors (11436): sensorservice died [0x205d050]
D/SurfaceFlinger( 119): Screen acquired, type=0 flinger=0x1a8d318
W/AudioFlinger(29202): power manager service died !!!

Hopefully this helps diagnose the issue. Please ask if you want the full log- looking at it, this seems to be the relevant bit. You can see me sleeping the device with the power button, then it reboots.

[Edit] I should add that it's basically been sitting on my desk at work all day doing nothing- a few texts sent and received and Exchange mail being pushed to it, that's all. I'd just sent a text and turned it off.

Someone else had the same issue on an AOKP ROM for the HD2: http://xdaforums.com/showthread.php?t=1441462&page=460

Seems the dev was working on it but I didn't see if he managed to fix it.