Well the question is about potential PS3 audience in the future, LBP is certainly a game that could capture some new gamers to the PS3 but the price of the console and lack of offering for the casuals makes me believe that not that many people out the will like this title.
Especially when there is the Wii with countless casual titles, that also offers great platforming: SMG, De Blob, Wario Land, Boom Blox to name a few.
Not all PS3 owners in others will like this game, PS3 is strong in this region, yet many gamers with this system are only interested in those big bad mature PS3 games, my friends with PS3 have GTA IV, MGS yet they havent heard of the LBP. I know that anecdotal evidence means nothing, but those friends fit the pattern of main group of PS3 users.
Install base of the Playstation 3 will make it impossible for a game like LBP to pull of numbers of Halo or Galaxy. I think LBP will be Successful without a doubt, but people overestimate it's legs (it wont have Nintendo type of legs) and i see it selling 3 milion copies LTD. It cant go over MGS4 or GTA IV numbers.