jailBREAK po hehe..
sa IOS 4.2/4.2.1, na update na yung baseband to 5.14/5.15, which hindi mauunlock ng ultrasnow, unless madevelop ng jailbreak team ang unlock for that baseband.
man, there has been significant updates on that from the devteam recently.
my iPhone 3G is now running on a jailbroken iOS 4.2.1, with an unlocked baseband of 06.15.00 (same as the iPad). i used the latest redsnow for the jailbreak, and then ultrasnow (from cydia) for the unlock. Works like a charm!
although, i think the iOS 4.2 experience is made for the iPhone 4's hardware.
i've noticed a small bit of lag in my iPhone 3G. hehe lugi yung hardware specs nito sa iPhone 4! Pero yeah, iOS 4.2 is unlocked already.