Apple has just seeded iOS 4.2 GM (build 8C134) and SDK (build 10M2423) along with the updated version of Xcode to the developers. iOS 4.2 GM and SDK is available for iPhone 4/3GS/3G, iPod Touch 4G/3G/2G & iPad (Wifi/3G). Probably this is the last build seeded to developers before Apple will release iOS 4.2 final which is due within this month.
iOS 4.2 beta comes with a few additional features like now with multitasking bar you can change volume and brightness at the same time, AirPlay is back to Youtube app, support to create free Apple ID in MobileMe plan plus there is a lot more improvement in performance as the devices run more smoother now.
The first beta of iOS 4.2 was released on Sept 15th, beta 2 on Sept 29th and 3rd beta was released on October 13th. Just like other betas, this beta build is available for developers only, so if you have a registered Apple Developer account, you can download the update by signing into iPhone Dev Center.
Update1: Jailbreak iOS 4.2 GM on iPhone , iPod Touch & iPad with Redsn0w 0.9.6b2
Update2: Download iOS 4.2 GM 2 Seed for iPhone, iPod Touch & iPad
Update3: Download iOS 4.2.1 GM Seed for iPhone, iPod Touch & iPad
Update4: Download iOS 4.2 (4.2.1) Final for iPhone, iPod Touch & iPad
Update5: Download iOS 4.2 Final for Apple TV 2nd Gen