I agree with the above post. As soon as net speeds as a whole make it practical to dl a 6-15gb game from a server, where we get the game quick, and the retail vendors have the power to serve the masses via internet we will see this implemented. Its already trending that way. Remember not everyone is on Fios, or Lightning, and most purchases are impulse. Waiting 2-3hrs for a game to dl is not impulse. This is why I dont think we have seen this en mass yet. Great idea though.
For games that you already own, you could stick your disk in, Sony would validate it, and you would be allowed a backup through psn with an exp. date, when the backup expires, you reinsert the disk, and it unlocks again for another 30 or so days. Sure as hell beats putting the disk in every time to play the game, or being broken by the kids. I think its a great idea if something like this could be done.