It was expected some time today. But according to the latest internet rumors, iOS 4.2 has been delayed (some saying for a week!) due to issues with wifi connectivity! iOS 4.2 for the iPad will bring a lot of changes, the biggest of which is multitasking, which came to the iPhone not too long ago.
Wifi issues came up with the GM build Apple released on November 1st. From what we’re hearing, Apple may release a second GM build as early as tomorrow to deal with WiFi issues. This will delay the release for at least a week while it is tested, but may not get public release until the following weekas it appears Apple will be testing it vigorously. iOS 4.2 should, however, come out before Thanksgiving. Quoted/ source