I too am sorely disappointed that the unlimited plan won't be offered. The ability to start, then stop, then start 3g service was a selling point to me as well.
I average about 2GB on my HTC Hero on Sprint barely doing any streaming (audio/video)--only surfing, emails, and my RSS feeds. I surf and RSS quite a bit with the device...to the point where I go through 2 batteries by the end of the day. On the other hand I'm pretty sure my usage on the phone would go way down if owning an iPad.
I could imagine iPad data usage would be WAY above the 2GB i already use. I mean think about it: The iPad was made to consume media. How large is a graphic intensive website? I can imagine I'd be surfing on the iPad a whole lot more than on my phone, and I'd being more inclined to watch streaming video on it. If I'm actually visiting and experiencing the web in FULL glory, 2GB is nothing.
Strangely perhaps this may be a reason for Apple not to include Flash on their mobile products. AT&T might have thought it would be too demanding on their bandwith; imagine all those flash ads on websites, along with all the pr0n people would be watching.
With all the $$ they're getting, I still can't figure out why in the world they can't just upgrade their crap. Infrastructure is all laid out, all they need to do is upgrade equipment

. If that were to occur it'd be easier for them to justify the price they're charging... and like Verizon consumers would be willing to pay for the coverage/reliability/speed.