The iPhone Dev-Team, makers of PwnageTool and QuickPwn (unlocking/jailbreaking software for the first-gen iPhone and iPod Touch)
have, at long last, announced a solution for unlocking the Apple iPhone 3G. According to this post on the Dev-Team blog, project yellowsn0w will hopefully be available on New Year's Eve as freeware all dressed up in an easy-to-use GUI, a la PwnageTool.
You'll have to have an iPhone 3G with baseband version 2.11.07 or earlier in order to run the unlock, and you'll need to jailbreak your iPhone before unlocking it. The unlock will be available via Cydia, and therefore accessable to PC and Mac users.
On a somewhat related note, AT&T is now selling refurbished iPhone 3Gs. On the one hand, a refurb will save you $50 off the price of a new model (8GB = $150, 16GB = $20). On the other hand, you'll only get a "90 day or more" warranty with a refurb, as opposed to the full 1 year warranty with a factory fresh model. Seriously, that's what AT&T's saying, "90 days or more." Huh. These are, of course, "with two year contract" prices - you're not about to walk into AT&T with a buck-fitty and walk out with a contract-free iPhone, sorry.