iOS 4.2 hasn't even been publicly released yet, but the gold master candidate has already been jailbroken using Redsn0w. In fact, using instructions posted on Covering Web, it's been possible to jailbreak iOS 4.2's gold master since the moment it was released. The jailbreaking steps are relatively straightforward, although they do require you to have retained a copy of the iOS 4.1 firmware in order for the procedure to work.
Of interest to those who don't want to jailbreak but are interested in getting hold of the iOS 4.2 gold master before its official release, Covering Web also has instructions and download links available for updating to iOS 4.2 early.
Updating in this fashion requires you to completely restore your iOS device, so make sure you back it up using iTunes before you do so. After the restore is complete, you can use iTunes to sync over your apps, media, and data.
Update: Restoring isn't necessary. Instead of holding down Option and clicking "Restore" in iTunes, hold Option and click "Update," then find the iOS 4.2 file and select it. Now your device will be updated without having to erase it first, which should save a couple hours of syncing files afterward.
I can't vouch for the results you'll get from Covering Web's jailbreaking procedure -- I'm too chicken to jailbreak -- but I've been running iOS 4.2 GM on my iPhone 4 since a few hours after its release and have suffered no issues. If you're too impatient to wait for the official iOS 4.2 release and have a couple hours of time to kill while your device syncs over all of its data following the restoration procedure, then you really don't have much to fear from an early update -- the last several iOS gold master releases have all been identical to the later public release, and gold master releases don't require a developer ID for installation.
Please note that TUAW cannot help you if you munge your device by upgrading early or by jailbreaking; in either case, you are on your own.