with my pi/raspbmc the 1080p movie playback is indeed smooth but the menus stutter awfully. But the reason for that might be that the menus doesn't fully utilize the opengl es of the pi or something else (bad skin?) so I wouldn't blame the pi yet.
Waiting with baited breath for mine to turn up! :-)
However, it was very naive of the grandparent poster to think that slow CPU powered menu screens means that the hardware accelerated video is lacking.