Visual Studio 2008 SP1 and .NET 3.5 SP1 offer an extensive list of enhancements, but also of bug fixes, here are the links in case (like me) your looking for a specific fix:

http://support.microsoft.com/kb/950263/ - List of changes and fixed issues in Visual Studio 2008 Service Pack 1

http://support.microsoft.com/kb/951845/ - List of changes and fixed issues in Visual Studio 2008 Service Pack 1 for Team Editions 

http://support.microsoft.com/kb/950264/ - List of changes and fixed issues in Visual Studio 2008 Service Pack 1 for Express Editions

http://support.microsoft.com/kb/951847/ - List of changes and fixed issues in Visual Studio 2008 Service Pack 1 for the .NET Framework 3.5