Oooh, that is a difference. I used -lqt on my link line, but I only have one Qt library installed, the qt-mt one (libqt.so is a symlink to libqt-mt.so), so I ended up using the multithreaded Qt library anyway.
So if you are linking against -lqt, and libqt.so is not a symlink to libqt-mt, then that might be the difference. I don't see why, but it might be. Try -lqt-mt and see if that helps, if this is the case.