A proposal for a semi-automated Escrow mechanism

A transaction can be written that requires two signatures to spend it next.  You write a payment that requires the signature of both the recipient and the sender to spend it.  To release the escrow, you give the recipient the signature for your half, or the payee can return it by giving you his signed half.  There's no mediator in this simple case.  The recourse is to refuse to ever release it, essentially burning the money.