Since yesterday I had no intellisense for standard ASP.NET Server Controls. Today I found a solution:
http://forums.asp.net/t/1205528.aspx
Quoting from the blog of Mikhail Arkipov (and the same goes for VS 2008)
I blogged earlier that in VS 2005 we switched from using static XML schemas to automatically generating them using reflection on Web controls. XML schemas are generated on demand as you open pages in your Web site. They contain information about ASP.NET controls, custom and user controls. You can find generated files cache in
C:\Documents and Settings\[Username]\Application Data\Microsoft\VisualStudio\8.0\ReflectedSchemas
For Vista:
C:\Users\[Username]\AppData\Roaming\Microsoft\VisualStudio\9.0\ReflectedSchemas