Primetime was planned to be an addition to the new Xbox Live experience, a feature which will allow players to use their avatars to complete in a form of live interactive game shows.
The feature will no longer be making the launch of the new experience, but thankfully it has not been cancelled and will be arriving shortly! Shortly, in the sense of next spring (boooo). Microsoft have announced that the delay was a requirement in order to ensure that the launch of the new Xbox Experience goes as smoothly as possible.
Looking forward the the feature itself and any further news, keep an eye on ConsoleMonster for further information!