[shillery alert, I'm the CEO of MailChannels] - It should be noted that Matt Sergeant works for MessageLabs, who use many TurnTide devices in their network, so his comments here are well founded in personal experience. And it should be noted that MessageLabs is a kick-ass company that does a fantastic job at fighting spam.
The main difference between TurnTide and our approach is that TurnTide traffic shapes at the network layer -- much like a firewall does. The TurnTide approach worked very well to punish high volume senders a few years ago, by forcing down their window size and backing up packets within their own infrastructure.
Today, however, most spam is sent by zombies, the owners of which could care less what's going on at the TCP layer -- since it's not their network that is being abused by high packets per second. What they _do_ care about is getting their messages dropped into the queue quickly so that they can move on. So what's important today is to 1) identify the zombies at zero-hour and 2) hold their SMTP connections open for as long as possible.
MailChannels Traffic Control is a hybrid of a traffic shaper and a proxy, with some nifty connection pooling features that help to reduce concurrency to the mail server. We believe this design is better suited to today's spamming reality where zombies are prevalent, rather than high volume senders, and high concurrency is the enemy, rather than high traffic volume.
Our directly in-field experience shows that spammers abort their connections within ten seconds, rather than hanging on -- because the economic proposition of spamming is predicated on fast delivery to the queue.