What happens if I take my jailbroken iPhone to the AT&T store?

My dad wants to go get a new iPhone4 because he is gonna give his old one to his nephew… But the current one is jailbroken-by me, so what happens if the people see that it's jailbroken?

Added (1). Btw I want to keep my old iPhone, so I don't care if AT&T doesn't want it. I'm just asking if they're gonna say that i'm violating their contract or whatnot just cause i jailbroke it.

If you take it to the at&t store they will not want anything to do with it it had no value reset it.

Just un-jailbreak it first. To do that just connect it to iTunes on any computer and use the Restore command.

Just restore it before you take it in.