The iPhone Dev Team have released a new iPhone 3G/iPhone 3GS carrier unlock, taking advantage of a susceptible baseband that Apple itself is still supplying, and allowing the older smartphones to use any SIM with iOS 4.2.1. This new redsn0w version relies on baseband 06.15 still offered as part of the iPad's iOS 3.2.2 firmware; however, there are also some significant provisos that mean this method isn't for everybody.
For a start, it will definitely impact your Apple warranty. Unlike other hack methods, which are reversible, this new redsn0w system leaves no route to backtrack from the 06.15 baseband. That means Apple technical support will definitely be able to see that you've modified your iPhone.
It also means that there's no way to return to either an official firmware or an official baseband, leaving unlockers at the mercy of third-party custom IPSWs in future. Still, if you're not especially bothered about Apple support - or, indeed, are out of warranty anyway - then this is certainly a straightforward way to use any carrier with an iOS 4.2.1 iPhone 3G/3GS.