Why does my battery drain real fast when 3G is on?

On my iPhone 4, if 3G is on, I see the battery drops from 100% to 20-30% by the end of the day. I've got iMessage on, check email and browse a bit, make a few calls and send SMS. Nothing really CPU intensive.

However when I switch off 3G my battery still shows 88% full now after nearly a whole day's use.

The battery drain didn't happen till a few weeks ago when I moved to a new work-site where I spend the day. It also appears to behave itself when I'm home on the weekends.

Can anyone help me understand what's happening here? Thanks.