iOS 4.2 GM Released / DO NOT UPGRADE - Get your 4.1 SHSH ASAP!
Apple has released the 4.2 GM Seed on their developer site. While there is an untethered jailbreak in the works for this, and it is possible to jailbreak this OS using redsn0w and pointing redsn0w at the 4.1 iOS file, due to the fact that many key apps (one big one being Cydia) do not work on iOS 4.2, it is STRONGLY URGED that you DO NOT UPGRADE.
This update also includes a new baseband that will RE-LOCK any officially locked phone and there is NO ETA on when an unlock for this version will be released.
In short -- if you rely on an unlock, STAY AWAY FROM 4.2 GM. If you are not a developer, STAY AWAY FROM 4.2 GM JB.
Also, this means that your time to get a 4.1 SHSH on file is drawing to a close -- Get TinyUmbrella and save a copy of your SHSH for 4.1 ASAP if you want to make sure you're on the jailbreak train after 4.2 releases.