I was curious, though I am not on AT&T yet. I am planning on getting an iPhone, and wanted to see what calling plan I can get away with. I called my carrier, Cincinnati Bell, to see how much data I use a month.
I am a business person, and I do upload images to my site, to social networks, blog, send up video and actually waste time (sometimes) with idle pursuits with my cell phone. I would call myself a moderate user of data, and I run an internet business. Here are the stats:
April 2010-13.8 megs
May 2010-8.65 megs
YMMV.