Microsoft sent out a mail on Wednesday informing MSDN and TechNet Plus users that Windows Vista SP1 images as well as Windows Server 2008 (RTM) have been made available to subscribers. Upon checking however, I was unable to find the promised SP1 image on my TechNet subscription.
Microsoft had last week, backpedaled and decided to move the release for certain [limited amount of] customers and beta testers up from mid-March to mid-to-late-February. I imagine some subscribers have yet to see it appear on their download list but should all go according to plan, those images should be made available before March.
Up to 3 pre-requisites will start appearing through Windows Update for everyone else before mid-March, which is required before SP1 can be installed on Windows Vista. This is due to an update of the update service from within Windows that is required before SP1 can be installed.