Apple has just pushed final Gold Master version of iOS 4.2 to developers. Only members registered in the iPhone Developer Standard or Enterprise Program. You can go download the GM seed now. Here's some of the new features in the iOS 4.2 GM version.
AirPlay is back to Youtube
Bug fixes makes it faster
The first beta was released on September 15th, followed by the second beta on September 29th, beta 3 on October 12th. iOS 4.2 should go public in days.