Encouraging people to expose their private keys makes me nervous. They're "private" for a reason, and it seems to me the vast majority of private keys will, a few years from now, either be stored on a secure device which is designed to never reveal them or will be split.
Why would a merchant "wish to accept typed or scanned Bitcoin private keys as payment" ? They're going to generate a transaction immediately anyway, it seems to me making a payment that way just opens up more potential ways of getting defrauded (e.g. merchant keeps the private key in case more bitcoins are ever sent to the same address, or merchant sends change to the same address and then a few days later uses the private key to take back the change).
I don't see any privacy or transaction-fee-saving advantages, either; am I missing something?
(I do see the usefulness of importing private keys into your wallet, that's a different feature that I'd like to be in the next release).