Apple has released the final GM (Gold Master) version of iOS 4.2 to developers! Only members of the iPhone Developer Standard or Enterprise Program can get this update by signing into iOS Dev Center.
Download iOS 4.2 GM for iPhone, iPad, iPod touch and SDK
UPDATE 1: Just follow our step by step guide to jailbreak iPhone 4, iPhone 3GS / 3G, and iPod touch 4G / 3G / 2G on iOS 4.2 using Redsn0w. Posted here