I just upgraded from 4.2.1 to 4.3, and have been spending the better part of the afternoon trying to get my phone to work again. After the upgrade, my battery started draining at a rate of 10% per 30 minutes. That implies in 5 hours the battery would be completely drained. I have not installed any new software or changed any settings other than the iOS upgrade.
In addition to this, the screen refused to auto lock. It is set to 1 minute, but it never turns off. So I restored, factory reset, restored, without success.
A plain 4.3 install with no restoring of any data seems to work fine. The phone is fast and does not drain the battery, and it auto locks the screen. The moment I restore my data, the above behaviour occurs.
Oh... And when I tried to access the Phone app - you know, the one you REALLY need when calling people, it crashes instantly with this:
Hardware Model: iPhone3,1 Process: MobilePhone  Path: /Applications/MobilePhone.app/MobilePhone Identifier: MobilePhone Version: ??? (???) Code Type: ARM (Native) Parent Process: launchd  Date/Time: 2011-03-09 10:42:04.366 -0800 OS Version: iPhone OS 4.3 (8F190) Report Version: 104 Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x00000000, 0x00000000 Crashed Thread: 6
The only solution I had was to perform a plain install of 4.3, then manually recreate my settings / use Mobile.me to sync the data back. A restore of 4.2.1 failed as it asked me for a passcode which I have never set.