Some Sony guy said it will be huge, there was some thread about it and Sony wanting Sackboy to be their mascot. I would think a mascot game would sell huge (Halo above 8mil and Mario Galaxy above 7mil). Those are long term sales so we will need to wait for LBP but i see LBP being overshadowed by many games this holiday.
Also this thread in a way proves LBP was thought to have a huge opening or there would be no need for this. Where there's smoke, there's fire.