This hasn't been confirmed by anyone at Sony, but various sites are reporting that Sony is offering a free game to anyone that had the evil demo disc erase their memory cards.
If you have one of the faulty discs, call up the Sony support line at 1-800-345-7669, and you should be offered one of the following games:
Wild Arms 3
Parappa The Rapper 2
Gran Turismo 3
Sly Cooper 2
Hot Shots Golf Fore!
Again, this hasn't been confirmed, but it's worth a shot if you lost all your game saves isn't it?