It won't sell as well as some are saying it will, but there is no way in hell it'll sell terrible. People seem to have a problem understand what causes games to sell. It doesn't have to do with quality, strong age appeal, ect. It has to do with marketing, hype, and brand appeal.
Perfect example? Red Steel. Launch title on the Wii that had large hype behind it, a massive wave of advertising, and strong brand appeal to early Wii adopters who wanted a shooter. But the game had bad quality and may or may not have been targeted at a majority of wii adopters in the beginning who were core gamers looking for quality titles.
So let's look at LBP. Massive hype behind it, will get legitimate advertising(already is actually), and has built up a responsive brand. The game is going to sell, and it's going to sell well. It might not do 4-5 million as some are suggesting but the title will go platinum easily. Too many people try to allocate sales soley to quality or some shit like that. Trust me core gamers will buy based on hype, and if it turns out to not be so good, then it's legs will die off. Red Steel was an example that was front heavy then dropped off. Haze was another example. But when it comes to casual titles like that, especially ones that offer to the core as well, they will sell given said reasons. NOt to mention this probably will be a quality title.