Now i'm shure that was not the problem of QWT. I was getting this problem when i was using a precompiled version of QT ( qt+mingw downloaded from trolltech ).

So this time I downloaded a QT source package and compiled it with mingw myself. Then i compiled the qwt. The plugin loads without any problems.

qt_4.3.3 qwt_5.0.2 mingw_3.81 vista_x64