They wont find out and they dont care just put your AT&T simcard and it will work fine.
US carriers dont blacklist phones so even stolen or lost phones will still work.
Update to 4.0 and turn off data in the settings so you can use it just for calls, texts and wifi for internet.
If AT&T does its periodic scans and notices theres an iphone with no data on your account they might automaticly add the $15 data plan on it.
If this is true then everything is fine. Do az he says