I had the same problem and eventually tracked it down to the Enhanced zoom desktop feature of compiz.
I just went into compiz config settings manager and disabled the Enhanced zoom desktop feature which uses the win+c keyboard shortcut to centre the mouse on the screen and then re-enabled the win+c under amaroks global shortcut settings and it worked again.
I had the same problem and eventually tracked it down to the Enhanced zoom desktop feature of compiz.
I just went into compiz config settings manager and disabled the Enhanced zoom desktop feature which uses the win+c keyboard shortcut to centre the mouse on the screen and then re-enabled the win+c under amaroks global shortcut settings and it worked again.
Hope this helps.