I actually made sure to set email push to manual once I got it set up. I didn't want my data used up by checking for email all the time. All yesterday whenever I used my phone, I always saw the little wifi graphic in the top left (as opposed to the 3G graphic that shows when I'm off of wifi).
I did accidentally just use 3G a few minutes ago when I checked my email, and my work's wifi hadn't connected yet. Now I'm up to 313 kB. 😛
I suppose it's possible that the iphone takes a few seconds to connect to wifi when I first wake it up, and maybe the data usage is from those few seconds of loading a page before the wifi kicks in. But yesterday I always made sure that the wifi graphic displayed before doing anything. If no one else is having this problem, then obviously there's something going on on my end somewhere. I just wondered if there was some function behind the scenes that bypassed wifi and just used 3G or something.