LBP sales are impressive, given that there are only 17.6 million PS3s in the hands of owners. It's a casual game for a console which is still a bit expensive for the mass market, so once the $299 PS3s arrive next year, you'll see a huge "tail" of second and third-year sales.
The real importance of LBP is that it signals a huge change in gaming. Power is slipping away from the giant media monopolies, in favor of fan-based gaming, based on non-commercialism and sharing. Some of the designs created by LBP fans are incredible, better than any commercial designer could have come up with. This is a good thing for gaming, and the monopolies will just have to learn to accept the reality that gaming has its own vibrant, non-profit public spaces.