I'm honestly going to buy a PS3...just for this game. The video below shows off the games amazing physics engine and limitless possibilities of what you can do when creating your own maps.
Everything you encounter during the game's built in levels can be used to create your own levels, and through out all the videos, there's a lot you can do. This next video is a capture of Sony's Press Conference where they used Little Big Planet, you'll have to watch.
It won't be out until Oct/Nov of this year, and with the PS3 80gig dropping its price to $399, I'm definitely going to buy one now x.x darn you Little Big Planet...how I want thee.
Here's a 10min video showing lots of creation features and in the last minute is the sharing features. There are a lot of amazing features, shown here is only a small portion.
The Awesome G4 at E3 2008 hands-on demo of Little Big Planet