When I'm at home, like I am this week for Spring Break, my phone can stay at a full five bars of battery life the entire day (from the time I wake up to the time I go to sleep, about 16 hours later).
While I'm at school, where there is 3G streaming through the air, I notice that by the time I'm about to go to sleep for the day, my phone is always down to four battery bars. The only variable that changes is the 3G. I don't use my phone any more or any less at school.
I don't necessarily find it surprising that 3G seems to shorten battery life, I do find it interesting nonetheless.