Earlier quoted context omitted.
I know for sure that transmission of HTML and images etc (i.e. most of what you need for webpages) and email continue to work when you hit the limit.
So... not quite unlimited after all? I have to echo omh and steveklabnik, I frequently use over 500MB (though usually under 1GB) without doing anything "funny" (e.g., tethering, torrenting, or just generally being a dick to the pipe). Most of my usage is streaming videos and music - a "fair" enough use of an "unlimited" data plan I would say! It's good to hear that these new terms won't impact you materially, but rea…
I found some figures by the way, 90% of T-Mobile customers don't go over 500MB, and "a single digit minority uses 30% of the bandwidth". I worked out that if 91% of their customers use 500MB, and 9% use 3000MB, this change reduces their overall bandwidth usage by 31% (assuming all those 9% switch to using only 500MB... if they jump ship, TMobile save 37%), but only pisses off 9% of their customers. I'm sure financially it's a net win... PR-wise... not so much.
(Most of that 91% are "well below" the 500MB line, if we assume 250MB average then the savings are over 50%! I can see why they wanted to do this)
Edit: Noticed while searching for figures that they have backed down! "There will be no change to the data packages for existing customers for the duration of their contract and we apologise for any confusion caused"