Comment 21 for bug 437378

VLC has included "XLIB_SKIP_ARGB_VISUALS=1" directly into their code (and I believe VirtualBox too), because this is the only way right now to prevent Qt from allocating an ARGB visual for a window.
although this can sound crazy, there is no other way (and yet, why the hell a video player would ever need a translucent video frame ?)

of course using XLIB_SKIP_ARGB_VISUALS must be done for each app specifically, not for the session.
it's maybe an X problem, but then why only Qt-based video app ? Gtk windows with an ARGB visual + opengl work fine.

if you can't reproduce it (really ? starting cairo-dock in opengl mode and then opening Skype or Smplayer works ?), maybe there is some hope ^^