Bad Ash has jailbroken the correct.
First, you won't be able to leave AT&T without a contract or an unsubsidized phone, which greatly increases the cost of getting the phone ($600-$900 probably).
Second, even unsubsidized, it will still be locked to AT&T (they will not be giving you an unlocked phone, just not making you sign a contract -- you'll still need pay-as-you-go/month-to-month AT&T service).
Third, if they unlock or iPwn 2.0 release, like Bad Ash said, the 3G radio is not compatible with all 3G networks, like T-Mobile in the US, so you would be stuck on 2.75G/EDGE. I'm not sure whether the aGPS would work, since it hits cell towers for the "a" part (it might anyway?)
The only advantage to jailbreak would be jailbreak-specific apps, which means stuff App Store wouldn't allow - VoIP over EDGE, pr0n, multitasking apps sans-notification server, etc.