Apple just posted iOS 4.2 Gold Master build in the iOS Developer Center. You can go download the GM seed now. Look for updates on what's new in this (almost) final build here on MacStories in a few minutes.
Build number is 8C134. It's a 549MB download for iPad and, as usual, it's also available for iPhone 4, 3GS and 3G and iPod Touch 2nd, 3rd, and 4th gen.
Together with the OS new build, Apple also released an updated version of Xcode and iOS SDK.
A GM build means the development cycle is almost over and the OS is basically ready to ship. Gold Master releases usually come a week before the public release, which means we could see the public release of 4.2 next Tuesday.
Update #1: Basing on a 5-minute quick test, it seems like there are no new features in this GM build. As we expected, it's a bug-fix related build aimed at making everything faster and smoother.
Update #2: AirPlay is back to Youtube app.
Update #3: You can now create a free Apple ID inside a MobileMe plan, but the feature is not working yet: