Solved!
Well, at least I can build my Qt test project with Visual nmake/cl.exe.
The problem was in the generated Makefile.Debug file.
I created a test project in my installed Visual Studio 2008 Express and compared the MSVC build command with the qt Makefile build command. I then made following changes in the Makefile.Debug:
Removed alot of defines and added a few.
Changed include path from relative to absolute
Removed most include paths, just kept the necessary ones
Added a bunch of Windows static libs (kernel32.lib user32.lib just to mention a few)
Changed "-D" and "-I" to "/D" and "/I" for defines and include paths respectively. Dunno if that really is needed, but so it was in my MSVC project...
The question is how to create the Makefile.Debug with qmake so it works from the start?