I agree. It would be impossible to implement.
What constitutes a used game ? Do you tie the disc-ID to the console or to the account? Either way it will cause an uproar, imagine:
Play a game in your living room and find out you can't play it in your bedroom.
Or play it on one console, then have your kid take it to his room, log on with a different online ID and find that he can't play it.
There are probably a few ways to do it. XBL assigns 1 user key and 1 system key for each protected title. In PSN, a user can activate up to 2 home consoles and 2 portable consoles for gaming.
Presumably the game developers can also implement their own rules. e.g., The GT5 DLC track is tied to a PSN account. However, it can be played by the owner together with other PSN users in the same network session even if they did not buy the track.
Cars in the DLC are also tied to the PSN account. As a result, other PSN accounts on the same PS3 could not use the cars. Polyphony patched the game so that we can transfer the DLC cars to the in-game dealership. Now everyone on that PS3 can purchase cars from the local dealership.