Well, I really should have used the term Unlock, which means you can use it on a different carrier. Currently you can only get an Iphone on ATT. Jailbreak means you can install 3rd party apps on the phone. If you jailbreak an IPhone it is also unlocked so the terms can somewhat be used interchangeably.
I keep procrastinating on doing it because I keep hearing rumors that they are going to open up the Iphone to other carriers, but the court case regarding the monopoly is still pending apparently. {Shrug} I have a very good reason for not jailbreaking it, if possible, that I can't really go into here on the net.