I read somewhere (can't find it now ) that it has been discovered that under iOS5 the phone still uses 3G (for some things at least) even when it's connected to Wi-Fi.
I guess this would explain increased battery usage, and it would explain something that happened to me...
I was out of the country the other weekend so using Roaming Data. My plan includes 25MB Roaming Data per day. At ~6pm I got a text from Vodafone saying I was nearing my limit (I think they send them when you hit 80%) so I reduced the amount I was using the phone (was using Maps a lot). An hour later I was back on the Wi-Fi at the hotel, and stayed connected to that until we went out the following morning.
Just before midnight I got another text saying I had gone over the 25MB by 3MB - WTF
So, it seems like my phone was using 3G even when it was on Wi-Fi. Coincidently (or not) this was around the time my battery usage went bad. Up until then I'd been getting excellent usage.