I have reported this to Apple but thought I should post it here too.
I have picked up a problem with cellular data (iOS 5.0.1 9A405).
Sometimes when I turn cellular data off it respects my wishes and turns off the 3G radio, this saves huge amounts of battery BUT sometimes when I turn of cellular data it tells me it is off but it is NOT and I can confirm this by moving away from my wifi spot where it will give me a message "Network settings lost". Other times when it HAS turned off cellular data properly I can move away from my wifi zone and the "Network settings lost" message does not pop up. As far as I can tell this is what was draining my battery so rapidly, for the last 3 days my battery life has been great and I have changed nothing else on the phone.
What I do now is if I see my battery draining quickly I turn cellular data on, then I turn it back off and then I restart the phone and then the setting seems to stick. This does not happen all the time, for the most part it respects my wishes when I tell it to turn cellular data off. Maybe some of you might find an improvement of sorts by testing if this is the case with your phone too.
I have gone 29 hours on one charge so far and I have 38% battery left, enough to get me through the rest of the evening which will take me close to 34 hours on one charge. Until I realised this problem I was barely getting 7 hours on one charge.
I have NOT changed anything else I do on the phone or how frequently I do it. My mail still checks every 15 minutes, I send whatsapp and text messages as much as I like and I dont have to stress when receiving a call that my battery may run out. A 10 minute phone call barely uses 1%-2% of my battery.
I'm still watching very closely at the battery life and hope to never see the drain start again. I have a regular habit of making sure WIFI ON & CELL DATA OFF when I am at home and WIFI OFF & CELL DATA ON when I leave home and so far so good.
I hope iOS 5.1 addresses this for good.