LBP the last hope? What about R2, KZ2, GOW 3, and GT5? PS3's still got some legs and a little bit of hope left.
LBP should do pretty well though. I'd say anything less than 2.5 million lifetime would be considered not a success, considering the development time, and the amount of support this game has gotten from Sony.
But this a rare game that'll get better with time as new and more creative levels get published. Word of mouth will spread, and sales will be pretty steady for quite a while.
PSN - Chefsean2006