[Samsung Galaxy Player 4.0, 5.0] xterminus:
No Hardware mods. The above code resulted in the following in my dmesg

<6>[ 1207.038888] Parameters have been set as DEFAULT values...
<6>[ 1207.046140] -----------------------------------------------------
<6>[ 1207.050839] Information of Parameters
<6>[ 1207.054614] -----------------------------------------------------
<6>[ 1207.062599] - param_magic : 0x72726624
<6>[ 1207.065725] - param_version : 0x13
<6>[ 1207.069769] - 0. SERIAL_SPEED : 7
<6>[ 1207.073483] - 1. LOAD_RAMDISK : 0
<6>[ 1207.077908] - 2. BOOT_DELAY : 0
<6>[ 1207.081888] - 3. LCD_LEVEL : 97
<6>[ 1207.084413] - 4. SWITCH_SEL : 1
<6>[ 1207.087973] - 5. PHONE_DEBUG_ON : 0
<6>[ 1207.091895] - 6. LCD_DIM_LEVEL : 17
<6>[ 1207.096636] - 7. LCD_DIM_TIME : 0
<6>[ 1207.099345] - 8. MELODY_LEVEL : 0
<6>[ 1207.103233] - 9. REBOOT_MODE : 0
<6>[ 1207.106878] - 10. NATION_SEL : 0
<6>[ 1207.111616] - 11. LANGUAGE_SEL : 0
<6>[ 1207.114203] - 12. SET_DEFAULT_PARAM : 0
<6>[ 1207.118078] - 15. VERSION(STR) : I8315XXIE00
<6>[ 1207.122962] - 16. CMDLINE(STR) : console=ttySAC2,115200
<6>[ 1207.129529] -----------------------------------------------------

I figured it could be the usb port, so I cleaned it. It helped a bit, in that usb insertion/unplug events are detected all the time again, but didn't help with my problem.

I've used two kernel's at this point. 2012.03.05 Entropy and the kernel that came with PA 2.57 (which is 2.6.35.7). I've also tried a complete nandroid recovery I made from a "stockish" system back in April 2012. Didn't change behavior of device. I've also tried an nandroid I made of an ICS ROM I fooled with before Christmas. No dice either.

I'm currently back and running Paranoid Android 2.57, which I'm very pleased with overall.

Just to clarify, and because I've been fooling with this problem for a few days now, this is what's happening:

When Nothing is connected (No USB or AC)

* Short Power button depression does not start device

* Holding power button down for 5+ seconds the screen flickers on, and the "pre battery loading" animation shows for a second. Very rarely does the battery actually show. Before the actually battery graphic is actually shown, the device reboots and loops. As nothing is plugged in, I don't see why this sequence should even be loading. It *should* show the samsung logo, but it does not for some reason.

* Holding power down + vol up results in same behavior as above.

* Holding power down + vol down results in download mode.

When Plugged into USB.

* Bootloop, which is normal behavior I guess with custom kernels. I can still get into download mode.

When Plugged into AC

* Under normal circumstances I'd see the pre-battery loading animation then the battery status. Then holding down the power button would start things up. In my case, nothing happens at all. It just sits there, no startup animation or other activity unless I actually depress (and hold) the power button which results in a single loop as describe above. I can get into download mode though.

Behavior I've noticed which is otherwise odd:

* It (used to) behave flakey on detecting usb insertion. Sometimes it wouldn't detect the device, with even adb failing to detect it. Now it seems to work with cleaning up the usb connections. No other changes in behavior through.

* Powering down the device completely results in all the odd behavior described above. Restarting from a running android system works fine. Restarting into recovery works fine too, so the problem is in the cold-boot process.

* The ONLY way I get the device to boot up after powering down is to go into download mode. When I flash a kernel, which resets the device (warm boot?) As soon as it resets, I can hold down the power and vol-up to get into cwm recovery. If I let it continue I get a normal boot.

The only thing I can think of at this point is that possibly a (boot?) partition which normally isn't touched by flashing roms or an nandroid backup was messed up. I'm pretty good with android overall, but I don't know the internals like a developer would. Is this a possibility? Thanks for the advice so far.