Haha, I personally don't jailbreak - for three reasons:
1) I've broken my iPhone. More than once. If I jailbreak my iPhone, they won't replace it. As of now, they've replaced it instantly both times I've had issues (one, I kept getting a white screen of death. two, the ringer switch broke)
2) I like AT&T. Maybe it really is awful, but I really have never had an issue.
3) I like being legit. I don't want or need "free apps" and "cool screen backgrounds."
That said, NO ONE is going to jail for jailbreaking. NO ONE. I don't care if you walk in the Apple store and tell them your phone is jailbroken - it's just not going to happen, lol.
Who said it won't get replaced if your jailbroken? Dude, freaking restore your phone before taking to the Apple Store and that is it...WOW. I exchanged my old 3G twice (I simply restored before taking it to the Apple Store) and that is it. I like being legit? HAHA. You mean you like being plain and uncreative.