Problem: I'd like to have my bitcoins ready on an exchange so I can sell when the price reaches $x. Storing all my bitcoins on an exchange is unsafe but if I wait until the price is $x and then send, I have to wait for 6 confirmation which could cause me to miss my opportunity. I need to way to have my bitcoins on the exchange when I need them right away but not be at risk of not controlling my keys for a long period of time.This problem could apply to a number of scenarios.

Could we not have a way of sending a "pending transaction"? What I mean is that you would broadcast a transaction to the network that you want to send x bitcoin to [address] as pending. This transaction goes through the normal process of getting mined and gaining confirmations but the difference is that the receiver does not yet have rights to spend the coins. At some point in the future the sender could broadcast a [confirm] or [deny] message(signed with their private key) to the network. Once the receiver gets my signed [confirmed] message they can use the message the pending transaction to take ownership of the coins and credit the account immediately. Upon a [deny] the pending transaction is cancelled and all pending confirmations are removed.

Could something like this work? /u/petertodd could you please tell me why not?