I think this is a decent idea just so you don't have to return DVDs to the rental store. But, this idea has already been attempted and failed really bad. (It was called DivX, before the video codec format of the same name came out) We'll see if this takes off.

The difference between this and DivX, I think, is that this is a physical expiration. The DVD medium actually reacts to its environment and slowly but surely dies, while the DivX scheme I think was software based, adn your dvd player had to support DivX to play any DivX dvds.