Do you live in an AT&T coverage area? If not, be careful how much of your time you spend roaming. I don't know how long it takes them to notice, but when I was in school, I was on a AT&T family plan even though I was the only one technically in a coverage area (my family is from midcoast Maine and not only did I have bad service from U.S. Cellular in the Boston area, but we were sick of being paying for U.S. Cellular or Unicell's awful plans). It worked out great for a while until AT&T saw how many of our minutes (mom+dad+sister) were out of coverage area and kicked us off. If we didn't leave immediately, they said they wouldn't port our numbers either (sounded just like a threat to me, but we switched to T-Mobile soon after that).
I think it took a couple years for them to find out. Now that I think about it they may have waited until we were no longer under contract to do it, I am not sure. Just a warning though. I know when I get an iPhone sometime after the 3G comes out I plan to move to my own cell plan because it would really suck to buy a 3G iPhone and then get kicked off AT&T.