The beta of Windows Vista Service Pack 1 was expected to be released last week. However, it did not happen

Some sources say the beta was only supposed to be available for “a select group of testers.”

A more official answer to this mystery is that Microsoft made a typo to the emails that they have sent to developers.

Instead of Vista SP1, Microsoft has released an updated Windows Driver Kit for Windows Server 2008. The article from Arstechnica’s site says that an update to the driver kit both Sever 2008 and Vista should have been released since they share the same Windows Driver Kit.

The final release of SP1 is scheduled for early 2008.

Source: Arstechnica