That is how it works:

1. QtCreator is not build with mingw, it's build with msvc. Someone even reported the stupidity to Nokia : http://labs.trolltech.com/forums/topic/1068
2. Download QtCreator sources and build them in release mode
3. Overwrite SDK QtCreator files with build files
4. Overwrite dlls in bin folder with dlls from qt/bin folder - so they're compatible with the newly build QtCreator version
5. Now your Qwt plugins work without a problem!

Yeah, I've wasted only this whole day for this :/