To reproduce:

Configure qt with the option to create the visual studio project file - makespec win32-msvc2008 - when attempting to launch visual studio by either double clicking the qmake generated .vcproj file or by calling devenv <filename>.vcproj on the command line, Visual Studio does not recognise the solution file as valid. The file can be opened by opening Visual Studio and using its file->open project menu, however this does not work consistently and some projects may be reported as missing.

This has been reproduced using Qt windows commercial prebuilt binary distro vs2008 version. It appears that the .vcproj file is not correctly formatted and so is not recognised by visual studio 2008.