[more shillery, I was a very early employee at TurnTide]
The TurnTide/SMS8160 is more than capable of handling zombies effectively. High concurrency has always been the enemy and the SMS8160 has always done both bandwidth _and_ connection shaping on ingress traffic. Since day one, way back in 2002, when it was called SpamSquelcher.
While Matt's points are salient, MailChannels' solution is in no way new or revolutionary, believe me. I've been doing this longer than anyone. I'm not knocking the solution, as I have no experience with MailChannels' software, just the characterization that its somehow groundbreaking.