RPGs are, by nature, heavy on the pre-orders, and sell a larger proportion of their total units on the front-end. LBP is a legs title, if there ever was one. It will sell for eons.
Don't kid yourself, If LBP is pretty decent, it will outsell Fable 2 by a mile, even if Fable 2 is phenominal (which I think it will be), just due to market appeal. RPG gamers are a smaller crowd than most hardcore gamers like to imagine.