iPhone unlocking is a little more cash-intensive now that Apple and AT&T are all contract-happy with the 3G, but it looks like would-be unlockers will have a little more time to save up those pennies -- the erstwhile hackers on the iPhone Dev Team say that while a forthcoming version of the sadly-named Pwnage tool will activate and jailbreak the iPhone 3G, unlocking is taking a little longer than expected, and they don't want to release the code until it's exactly right.
The tool still unlocks first-gen iPhones running 2.0 and allows jailbreak apps to co-exist with the App Store, so if you're desperate to unlock right away, we'd say a first-gen unit is your best bet.