I doubt they'd fine you. If they were out to do that, a whole lot of people would get fined, because a whole lot of AT&T users install a warez'd copy of Win2K AS or the like (just to feel "l33t") and stupidly leave IIS checked (and subsequently get their boxen jacked by CodeRed/Nimda).
I've been running an FTP/rsync server on AT&T broadband for months now, and AT&T doesn't seem to care. My theory is that they're pretty lax about how you use the bandwidth you're given, as long as you don't run some service that disrupts the AT&T network (like a DHCP server). Try to steal yourself some extra bandwidth by uncapping your cable modem, though, and they get pretty pissed off--I've heard of them permanently terminating service and actually blacklisting uncappers for that sort of thing.