I've just claimed my iPhone warranty regarding intermittent no service issue (especially when no 3G signal around), the iPhone heat up issue, battery drain (which is caused by no service and heat up issue, ends up only 4 - 6 hours when it drains so quick). I've tried all iOS (5.0.1b, 5.1, 5.1.1), reset everything (network, all content), DFU recovery, or any other suggested steps written in the web -- which seems to fix the problem, but at the end of the day, the problem goes back.
Now, I got an iPhone replacement -- it takes more than 2 weeks to get it, and for the last 1 day, the iPhone looks very promising.
Current status: 5h 55m, standby 1d 1h and I'm on 44%. On average, it's like ~2.2% drops per hour. I consider myself a light user while testing it.
It's iOS 5.1.1, build on May 2012 when I run serial number check. No 128K USIM available in my country and using 32KB SIM seems to be just fine.
The settings I used which some of them I found on the web:
Location Services only turned on Compass Calibration.
Notification enabled for services that I need (no weather, no stock)
Turned off "Send Diagnostics & Usage".
Turned off Bluetooth
Turned off automatic date & time adjustment.
Turned on PING restriction.
Turn off iCloud for everything (only turned on the contact when I need to update contacts), no mail setup yet (I'm using Sparrow).
Vibrate only on silent. Brightness ~40%.
Wifi ON, 3G ON.
I recall my first month with the iPhone was just fine.. Probably there was an occasion where it gets very hot, the phone somehow went bad, and it's irreversible