One more point I'd like to make is that the _only_ person who can double spend is the owner of the coin (since only he knows the private key). The only time clients see a double spend is if the owner is purposely being malicious. It is therefore reasonable to drop _both_ the original and the second transactions if a double spend is seen within a window of time.
But dropping the transaction penalizes the recipient, not the sender, right? It's bad enough that the second recipient gets screwed out of the bitcoins. Why would you want to double the amount of innocent victims, by also penalizing the first recipient?
I think you are trying to hurt the sender by locking up the coins, but I'm not sure your proposal achieves that.
(then again, I'm still a newbie, so pardon my ignorance, if I'm wrong!)