Just an fyi...After roughly 2 weeks of iPad 3G ownership, I have transferred almost 200GB in data over 3G under what I would consider light use conditions. It would seem difficult to me to sign up for 3G and not exceed the 250GB limit if one uses 3G daily, even just a little bit.
EDIT: Oh for Christ's sake, I forgot the major caveat: I don't have wireless in my office, so it is on 3G most of the work day. But during that time, I am not actively surfing, just receiving mail and push notifications...and the occasional Words with Friends. So not heavy use, but certainly may affect my conclusion above. Fucking migraine.