I believe that unlock tool for iOS 4.2 and iOS 4.1 on certain devices is almost ready and iPhone Dev Team is waiting for the release of iOS 4.2 before they make it public.
Chpwn, who is an active member of iPhone Dev Team, just updated his single-page website about unlock. The page reads that iPhone 3G and iPhone 3GS users will soon get an unlock for iOS 4.1 and iOS 4.2.
The users of other idevices like iPhone 4, iPod Touch 4G etc will have to wait litter longer for iOS 4.2 and iOS 4.1 unlock. Sherif Hashim has already reported an early initial 1st step success on both iOS 4.1 and 4.2 basebands (05.14.02, 2.10140).
Those devices which have never been past iOS 4.0.2 can be unlocked using Ultrasnow. And if you updated your device while preserving the baseband with Snowbreeze or PwnageTool, you can also use the latest Ultrasnow to unlock iOS 4.2 GM and iOS 4.1.