I think LBP will make 3-4 million someday.
Remember, it is a good game, so the more people that buy it, the more word of mouth there will be. Should have steady legs, hinder only by the PS3 smaller userbase.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut