This problem was found using Qt Linguist from several binary installers from releases.qt-project.org, e.g. http://releases.qt-project.org/qt5.0/beta-snapshots/2012-08-07_12-43-19/qt-linux-opensource-5.0.0-beta1-offline-x86_64-ubuntu1204.run. The problem appeared in both Digia-generated and qt-project-generated installers.

After launching <InstallDir>/Desktop/qt/5.0.0beta1/gcc_64/bin/linguist, The Ubuntu taskbar and menubar vanish and do not repaint themselves even when the screen is locked and unlocked. The taskbar and menubar are both sitll present and functioning, as the taskbar tooltips still work and clicking in the top right corner of the screen still opens the system menu.

Even after quitting Linguist, the taskbar and menubar still won't repaint and I couldn't find a way to recover without rebooting the virtual machine.