MacStories has heard from a reliable source who confirms the last-minute problem.
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 week as it appears Apple will be testing it vigorously. iOS 4.2 should, however, come out before Thanksgiving.
iPhoneHellas, who has been accurate in their previous predictions, says the firmware will be released on November 16th.