Until the spammers build in retry into their bots, I'm a firm believer of greylisting.
They have. I'm a sys admin at a major hosting provider, and I've seen it in action on at least one customer's box who was using greylisting. Considering spammers have near-infinite resources, it was only a matter of time before they'd either retry delivery on the same message, or simply wait an hour or so and try sending a new message.
