This doesn't make any sense to me: why the iPhone doesn't automatically switch 3G off when connected to a wireless network?
I always thought iPhone did that since the very beginning and my battery lasted 4–6 hours on average per day. (I can almost swear that I remember Steve Jobs himself telling that in the stage back then)
Then a friend of mine asked me why I don't switch off the 3G when I am in the office to save some battery instead of charging it at my desk so in case I had to get out it was fully charged back again.
So, I tried... Boy, it was a huge difference! My iPhone's battery will last 10–13 hours on average. That means usually I can get back home from work and have my phone still working in my pocket.
EDIT: my question is about worse battery performance when 3G is enabled. (Even though not supposed to be used) And I believe this isn't related to me, but to any iPhone user really.