Low tech solution for instantaneous payments, no computer needed.
You simply print out the private key that holds a certain amount of bitcoins. You give this paper to the receiver, who then transfers it to his wallet at the "point of sale" computer.
We are back at the original double spending problem, so the payee still has to wait for confirmations (when he transfers to his address). However, at least the payer does not need access to his bitcoin wallet at the moment of transfer. Also, only works for fixed amounts, similar to prepaid phone cards.
Also, if you send money in email, at least bitcoins are not lost forever if the receiving side never uses them.
Any value in this?