I came across this link on the DOTNET-CLR mailing list today. It points to a GotDotNet workspace where Peter Hallam has posted a library containing interop declarations for many, many of the APIs from Windows.h in [DllImport] form. Handy!
Apparently, he has some sort of semiautomatic process for producing this file. If you head over and bug him, he might post the code that he used to create the declarations.