If you've followed the "tips for improving your battery life" thread, then you've probably seen me post. I picked up my Thunderbolt on launch day more than a week ago, and here is what I have experienced.
The first two to three days I got around 4 hours of life after adjusting settings. My settings remain the same...
Screen - 1% (dimmest it can go)
Syncing - Only emails arrive instantly, friendstream is off, weather is 30 minutes
Widgets - Battery life, HTC clock/weather
Installed the app that kills apps on starup (not task killer)
Middle of last week though, something changed. I was getting over 12 hours of charge with 4+ hours of screen on time plus talking. wow! I was doing this and living the good life.
Two days ago however, it all went back. I have changed no settings, go no new places.. and always on 4G as usual. Now I'm at 3.5 hours of battery life, if I'm lucky. I'm at 75% battery life right now with 1.5 hours since unplugged, 7 minutes of screen on time and 2 minutes of voice calls.
Defective battery? Possibly. I hope it's not a defective handset, because I really don't want to start all over with apps (mostly angry birds.....)
Hopefully my problems can help you. Good luck.