Apple released iOS 4.2 GM a few hours ago, and the word on the street is that you can already jailbreak it using the latest RedSn0w update. Like MuscleNerd suggested, unless you’re a jailbreak app developer or unless you know exactly what you’re doing, it might be a good idea to stay away from it.
Remember that updating your iPhone to iOS 4.2 GM might prevent you from unlocking your iPhone forever (most likely not, but you never know)…
Today Apple released to developers the GM seed for 4.2. Tinkerers will find that yesterday’s redsn0w jailbreaks today’s 4.2 GM seed, simply by pointing redsn0w at the 4.1 IPSW (rather than the 4.2 one). Right now it mostly only makes sense for JB app developers to do that because many apps (including Cydia itself) need to be updated for 4.2. However, if all you want to do is enable afc2 (to use iFunBox or other file browsers), or to tweak settings like Battery % and Homescreen wallpapers, then go for it (if you have valid paid access to the GM seed). Be sure to uncheck the Cydia box, though! Ultrasn0w unlockers should stay very far away from this!!
Proceed at your own risks, but make sure to let us know how it goes if you decide to go for it!