iOS 5 Upgrade: Massive Battery Drain and Crashing Phone App
Turn off Wi-Fi Sync.

I did it and voilá!

It seems that iPhone is constantly looking for an iTunes and tries to connect to it permanently. In fact If I had WI-FI Sync on, iPhone would appear on iTunes for a couple of seconds and then disappear.

To turn it off just plug your iPhone to your computer, open iTunes, click on your iPhone on the left bar and uncheck Wi-Fi Sync.

Good luck!