If you followed the interviews with Phil Harrison at GDC, it was pretty clear that the game itself is going to be released on the PSN this autumn. From what I gathered, it won't be merely a demo, but a full commercial release. The difference between it and the Blu-ray release next year is that the Blu-ray version will incorporate some of the user-created levels made in the time between the PSN release and the Fall release.

In other words, I think LittleBigPlanet is actually being released in full in the Fall on the PSN. You can't really say the Blu-ray release will be any 'fuller' since it will only be incorporating content or updates that will already have been available to the community online. I think the Blu-ray release will really just be for people who are not online or who would prefer to have content on a disc rather than having to download levels.