A "Windows Vista SP2 Prerequisites" update is coming via Windows Update on April 28, 2009. Showing that Service Pack 2 is expected early next month.

Update for Windows Server 2008 and Windows Vista (KB955430)

Locale: All
Deployment: Important/Automatic Updates, WSUS, and Catalog
Classification: High Priority, Non-Security
Target platforms: Windows Server 2008 and Windows Vista
Approximate file sizes:
Windows Server 2008 for IA-64 update: ~ 14268KB
Windows Server 2008/Windows Vista update: ~ 4287KB
Windows Server 2008 x64/Windows Vista x64 update: ~ 10148KB