can ATT find out that i unlocked my iphone?

[Dave]

I have a regular 3gs iphone with ATT contract right now, but i am leaving to another country for four months and i am going to unlock my phone so i can use it over there. But i am gonna be back in fall, so i was wondering if i come back with my unlocked iphone and put my regular att number in it that has the contract and everything will ATT be able to figure out that the phone is unlocked??? and if they do what can they do about it???? Thanks