I heard a rumor that the streaming service they're integrating with would be capable of streaming every PS1/PS2/PS3 game that is currently on the Playstation Store. Even better, I heard that all of those games would be FREE for PS Plus subscribers. No source, since I heard it from a friend of mine at work, but that seems like awfully wishful thinking.
That would make PS Plus the most cost effective subscription in the history of the world, considering PS1/PS2 games retail for about $9.99 on the store, and PS3 games can be up to $60.