It should of been out already, but there were still bugs in the ipad version, which is why they released a 2nd gm build friday. For those overly eager, the GM builds don't require that your device be registered as a developer like the normal beta builds of iOS do.
I've been using the GM builds on both my iphone4 and ipad, and while it's really not all that big of an update for the iphone it is HUGE for the ipad. I've been waiting for multitasking for a while
Airprint was the only other major new feature, and it's been neutered at the last minute... as of now it only works with like 3 HP printers, I expect that there will be a software update to enable printing to shared printeres shortly after iOS 4.2 actually ships. (There already is a hack to re-activate it on OS X 10.5.5, the necessary code is already in the OS update and just needs to be activated.)