You already (should) know that your 'unlimited data' isn't actually unlimited in terms of sheer bandwidth -- every carrier has either official or unofficial data caps. Transfer more than, say, 5 gigs of data and you might be looking at a nasty call and an unceremoniously cancelled account. Add another limitation on to that there 'unlimited' plan: the type of data you're allowed to send or receive:

[AT&T] told the FCC on Friday that if they catch you using their mobile broadband network for P2P, they'll nuke your account. Specifically, senior VP of dealing with the government Robert Quinn said that “AT&T's terms of service for mobile wireless broadband customers prohibit all uses that may cause extreme network capacity issues, and explicitly identify P2P file sharing applications as such a use.” - Gizmodo

Exactly what qualifies as “P2P” is sadly (and expectedly) vague -- just apps on the phone, P2P on your laptop via a tethered connection, all are likely up for grabs. We're guessing that AT&T is feeling the pain of a 3G network that doesn't have the capacity for serious, heavy use by a large population, especially on the upload side of things. Yes, we probably have the iPhone to blame thank for AT&T sudden vulnerable feelings.

Thanks to Rene for the tip!