The only way to hit the limit would be to tether the phone to a laptop and download movies non-stop.
That's what the limit is for, it IS unlimited for cell phone users. There's no way you could hit the cap just surfing the web. But it helps them stop people who tether and abuse it.
(Note that I say abuse. If you tether and use it reasonably you should still be ok.)
I've heard different amounts for the cap, but it's somewhere between 5 to 7 GB. For comparison, for my first full billing cycle I used about 500 MB of data. There'd be no way to hit that limit using just the phone.
(I did use wi-fi half the time, so if you were on the call network ALL the time you might hit 1-2 GB if you really, really tried. But that would be hard to do.)