yep had this problem briefly with my 2010 15". I would set the background and it would be shown as changed in the settings, but no change would occur. A restart fixed the problem. Had it happen to me couple more times and since then its been fine.
This ranks right up there with the instant screensaver which just randomly kicks in mid work (or even mid sentence- just did it to me as i was typing this), no matter what you change the settings to or delete the screensaver plists. Clean install is in order I believe.