Multimedia keys don't work for Clementine on Kubuntu 12.04. Yeah. It's a Clementine 1.0.1 bug, which still has to be fixed by the developers.
Follow up the issue here for a workaround (which still has to come) - http://code.google.com/p/clementine-player/issues/detail?id=2908.
Or maybe a patch will be issued soon, and will be delivered through an update.