It's not unlimited, it is "unlimited" which means that you can go a little bit past 5GB once in a while but if you use several GBs over in a month or consistently go over, they might cut you off or bill you.By the way, a friend at an AT&T store said that if you go above 5GB a month (no matter how), you'll get a stern warning, and then if done again, a huge bill or cut off. What happened to unlimited data use?????
Cellular data is on a finite shared network going through the same towers as the voice calls even if they are on different network layers. If everyone started using like crazy, the whole network could crash including the voice calls layer.
Also, I'd like to point out that even though I have a 6GB plan, I have only used about 70MB so far after having my iPhone 3G since July 11th.