[Galaxy S I9000 General] powerdams:
1. You can restore if you make a "nandroid" backup first:

http://xdaforums.com/wiki/index.php?title=How_to_backup_or_recover_via_Nandroid

2. Yes after the flash of JVT you need to put semaphore_kernel_JVT_1.8.2bm.tar as PDA in Odin.

EDIT:

I upload my new build.prop for JVT, if someone want to try ; )

http://www.multiupload.com/PAFAWZMW7I

There is some tweaks: 3g/battery/responsiveness... etc

You can edit your build.prop like this one or just copy and paste in /system with root explorer.

# begin build properties

# autogenerated by buildinfo.sh

ro.build.id=GINGERBREAD

ro.build.display.id=GINGERBREAD.XXJVT

ro.build.version.incremental=XXJVT

ro.build.version.sdk=10

ro.build.version.codename=REL

ro.build.version.release=2.3.5

ro.build.date=Thu Sep 29 11:43:43 KST 2011

ro.build.date.utc=1317264223

ro.build.type=user

ro.build.user=se.infra

ro.build.host=SEI-22

ro.build.tags=release-keys

ro.product.model=GT-I9000

ro.product.brand=samsung

ro.product.name=GT-I9000

ro.product.device=GT-I9000

ro.product.board=GT-I9000

ro.product.cpu.abi=armeabi-v7a

ro.product.cpu.abi2=armeabi

ro.product.manufacturer=samsung

ro.product.locale.language=en

ro.product.locale.region=GB

ro.wifi.channels=

ro.board.platform=s5pc110

# ro.build.product is obsolete; use ro.product.device

ro.build.product=GT-I9000

# Do not try to parse ro.build.description or .fingerprint

ro.build.description=GT-I9000-user 2.3.5 GINGERBREAD XXJVT release-keys

ro.build.fingerprint=samsung/GT-I9000/GT-I9000:2.3.5/GINGERBREAD/XXJVT:user/release-keys

# Samsung Specific Properties

ro.build.PDA=I9000XXJVT

ro.build.hidden_ver=I9000XXJVT

ro.build.changelist=617736

ro.flash.resolution=720

# end build properties

#

# system.prop for smdkc110

#

rild.libpath=/system/lib/libsec-ril.so

rild.libargs=-d /dev/ttyS0

ro.sf.lcd_density=240

dalvik.vm.heapstartsize=8m

dalvik.vm.heapgrowthlimit=48m

dalvik.vm.heapsize=128m

windowsmgr.max_events_per_sec=150

ro.opengles.version=131072

# Samsung USB default mode

persist.service.usb.setting=0

#

# ADDITIONAL_BUILD_PROPERTIES

#

ro.opengles.version=131072

ro.config.ringtone=01_Minimal_tone.ogg

ro.config.notification_sound=01_Sherbet.ogg

ro.config.alarm_alert=Good_Morning.ogg

ro.url.legal=http://www.google.com/intl/%s/mobile/android/basic/phone-legal.html

ro.url.legal.android_privacy=http://www.google.com/intl/%s/mobile/android/basic/privacy.html

ro.com.google.locationfeatures=1

ro.setupwizard.mode=DISABLED

ro.com.google.gmsversion=2.3_r7

ro.HOME_APP_MEM=256

ro.PERCEPTIBLE_APP_MEM=256

ro.SECONDARY_SERVER_MEM=256

ro.VISIBLE_APP_MEM=256

ro.HIDDEN_APP_MEM=256

ro.FOREGROUND_APP_ADJ=0

ro.VISIBLE_APP_ADJ=1

ro.SECONDARY_SERVER_ADJ=1

ro.HOME_APP_ADJ=1

ro.HIDDEN_APP_MIN_ADJ=22

ro.CONTENT_PROVIDER_ADJ=23

ro.EMPTY_APP_ADJ=25

wifi.interface=eth0

wifi.supplicant_scan_interval=90

debug.sf.hw=1

debug.performance.tuning=1

video.accelerate.hw=1

ro.secdevenc=true

keyguard.no_require_sim=true

ro.com.android.dateformat=MM-dd-yyyy

ro.com.android.dataroaming=true

ro.ril.hsxpa=2

ro.ril.gprsclass=10

ro.ril.hep=1

ro.ril.enable.dtm=1

ro.ril.hsdpa.category=10

ro.ril.enable.a53=1

ro.ril.enable.3g.prefix=1

ro.ril.htcmaskw1.bitmask=4294967295

ro.ril.htcmaskw1=14449

ro.ril.hsupa.category=5

ro.ril.disable.power.collapse=0

ro.telephony.call_ring.delay=0

ro.media.enc.jpeg.quality=100

ro.lge.proximity.delay=25

mot.proximity.delay=25

ro.config.notification_sound=OnTheHunt.ogg

ro.config.alarm_alert=Alarm_Classic.ogg

media.stagefright.enable-player=false

media.stagefright.enable-meta=false

media.stagefright.enable-scan=false

media.stagefright.enable-http=true

media.stagefright.enable-rtsp=false

net.tcp.buffersize.default=4096,87380,256960,4096,16384,256960

net.tcp.buffersize.wifi=4096,87380,256960,4096,16384,256960

net.tcp.buffersize.umts=4096,87380,256960,4096,16384,256960

net.tcp.buffersize.gprs=4096,87380,256960,4096,16384,256960

net.tcp.buffersize.edge=4096,87380,256960,4096,16384,256960

dev.sfbootcomplete=0

dalvik.vm.dexopt-flags=m=y

dalvik.vm.execution-mode=int:jit

dalvik.vm.dexopt-data-only=1

dalvik.vm.checkjni=false

dalvik.vm.lockprof.threshold=500

dalvik.vm.stack-trace-file=/data/anr/traces.txt

persist.sys.use_dithering=0

persist.sys.purgeable_assets=1

persist.sys.composition.type=gpu

pm.sleep_mode=1

net.bt.name=Android