Neowin reader nX07 claims that a "friend of a friend who works for Microsoft" has first hand knowledge that a batch of Ultimate Extras will be released in late Q2 or early Q3 of 2008. Below is his post from the forums.
I brought up Vista Ultimate and its "extras", and he told me that because of the ''unexpected" flood of Vista issues Microsoft had to divert it's resources elsewhere and have SP1 cover more than internally-planned. Also, believe it or not, supposedly the Driver and Application Compatibility issues (Which technically was not Microsofts' fault and thus didn't need to do anything) made Microsoft very unhappy since it gave many software/hardware developers more than enough time to prepare, and basically forced Microsoft basically play hound-dog to all those companies.
He mentioned that come Late Q2 to Early Q3 of 2008, Microsoft will have a "wave" of Extras coming covering functionality, aesthetics, and even performance (seems a bit unfair if performance extras will only be for Ultimate users though).
Take it the way you want it, which is probably with a grain of salt, but you heard it first here folks!
Link: Neowin Forum Discussion