If you are a Mac user like me, you may be experiencing some OpenGL problems (no screen updates, no menus) under Mac OS X 10.4.3. From the reports it seems that this only happens on systems that have NVidia videocards. The problem seems to originate from the OpenGL drivers. Apple has been notified of the issue and hopefully it will be fixed in 10.4.4. In the meantime, people have reported a workaround: resize the Blender window to a smaller size and the menus will work again.
More information can be found on the Blender bugtracker and on the Blender.org forums.