There is some good news if you are the owner of one of the older iDevices like the iPhone 3G and iPod Touch 2G or iPhone 3GS (running the older bootrom). The iPhone Dev Team has noted that iOS 4.2.1 jailbreak for these devices is already ready for download.
This is thanks to the updated RedSn0w 0.9.6b3 that was released last week to jailbreak the GM build of the iOS. The Dev team says that the recently launched iOS 4.2.1 is primarily a public version of the GM build and so the same RedSn0w tool should work on these devices as well.
A word of note if you are also looking to unlock your devices. A new updated version of Ultrasn0w that will work on these devices (with the older bootrom) is expected to be released later this week. So unless you have an iPhone 4 or a 3GS with new bootrom, your device should be good to use by the end of this week.