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

Description:
Install this update to enable future updates to install successfully on all editions of Windows Server 2008. This update may be required before selected future updates can be installed. After you install this item, it cannot be removed. This update is provided to you and licensed under the Windows Vista and Windows Server 2008 License Terms.