Ouch! According to this thread on the Apple forums (via TUAW), AT&T users from Chicago to Milwaukee to St. Louis to Kansas City aren't able to get online via EDGE. That's a major black-eye for AT&T -- and of course it doesn't just affect iPhone users. Things are supposed to get running again by the end of the day ...unless it take until February 5th. I'm based down in Florida so everything's in the clear.
Update: Gizmodo tells us what's going on:
However, an insider who works on networks dropped me a line as to why AT&T's 3G network is having these sporadic issues today: apparentely, six GGSNs "rolled over". (GGSNs are gateways between wireless and regular networks.)