While the official public release isn’t due until sometime later in November, Apple has seeded paid iOS developers with a Gold Master build of iOS 4.2 for the iPhone 3G, 3GS and 4, iPod Touch 2nd, 3rd and 4th generation, and the iPad.
The newly updated Apple TV hardware is also likely to be updated to iOS 4.2 at some point this month or early in December, however as there is no Apple TV developer program right now, Apple is not providing early access to iOS builds for that device.
A new Xcode build and the corresponding iOS SDK is also available to paid developers.
As previously reported, the screen rotation lock button on the iPad is now going to be a mute button, which Apple mentions in the release notes for the iOS 4.2 GM seed:
The iPad screen rotation lock switch now functions as a sound/silent switch in iOS 4.2.