iPhone 3G/3GS owners who found themselves stuck with version 05.11 of the baseband (either by accident or because they bought it that way) are now in luck! geohot was able to turn the already-public at+xemn crash into an injection vector, which can be used to inject his version of the unlock. The blacksn0w unlock is available for free via Cydia by adding the repository http://blackra1n.com in the Manage->Sources panel. Congratulations, geohot!
Those of you who are already unlocked at 3.1.2 because you kept your 04.26 baseband now have an extra cushion of comfort, and more choices: ultrasn0w, purplesn0w, and now blacksn0w (and of course the original yellowsn0w too if you’re still back at FW 2.x). Whether or not you choose to update your baseband solely to use the new unlock is a personal choice, but so far there are no advantages to doing so (and remember you can’t come back to 04.26 after you’ve gone to 05.11).
As with all the unlocks, it will probably very soon be re-sold through scam sites that charge you money for what is offered to the community for free. Please stay vigilant for these scam sites and steer your friends away from them.
Update: Some commenters are reporting a lingering problem with WiFi while using blacksn0w. Some are able to solve it with a single “Reset Network Settings” but others say they need to do that periodically. So far there seems to be no pattern to those affected or the best way to fix it.