John,
Are you using EV1?
if so, then you automatically get FireSlayer.
While they dont' describe it, my guess is that it's something built into their routers that allows them to identify problem traffic and silently drop traffic from the IPs causing that traffic.
You can protect your servers to some extent with the APF (Advanced Policy Firewall), and we find it does help some of our clients.
but as DanB points out, it won't help much in a large DOS attack; you have to get your upstream to drop packets to protect against that.
Jeff