It was in Plus there for awhile. That and it was given away for free I believe when the network went down for a few months last year. Playstation felt bad about it being down so they gave free games away. Most people have moved on to playing LBP2 and LBPK. I believe they can still play and heart levels from LBP1 while playing LBP2.
Holly Crap, I just ACED AVALON last night. Where In The World Is Avalon Centrifuge. One of the HARDEST levels, ever to ACE. GOOD TIP for this level: Make sure you volume, is turned up, because you need to hear the guns before going off. You won't have much time to move. This level is pure HELL!!!