Hello
Ubuntu 11.04 here and same problem.
But I run Gnome classical desktop...

The problem is in compiz.
Launching ubuntu in classic mode without special effect will permit launching bitcoin in gnome.
Then, if you launch compiz with --replace, it will kill the classic/no effect window manager to replace it by itself : bitcoin still here.
Then, kill compiz : bitcoin still here
Then, close bitcoin with CTRL + Q and relaunch compiz with --replace.
Relaunch bitcoin... If you are lucky enough you will see it appear in a totally white window then disappear  with an animation like if it has been minimized, but problem, it is nowhere to see in the taskbar

Should be investigated