SQL Server 2005 Service Pack 2 was released yesterday, and I've just downloaded and installed the Express Edition with Advanced Services SP2 onto my Vista PC. Smooth sailing all the way, and no compatibility warnings. Hooray!
Here's a link to SQL Server 2005 SP2 on the TechNet site, and you can review this document on what's new in SQL Server 2005 Service Pack 2 for information on new or improved features in Analysis Services, Database Engine, Integration Services, Reporting Services, and Shared Tools.
Service Pack 2 will eventually be available for download over Microsoft Update (as reported on the SQL Express Weblog), but you can download it today for free.
I had trouble at first navigating through to the install links, so if you're just here for the download, here are direct links:
Hope that helps!