After much digging, I have come to the conclusion that the WRVS4400N is not compatible with either the native OS X Snow Leopard Cisco IPSec VPN client, or IPSecuritas (even though IPSecuritas even has a setup wizard for the particular model). Under firmware V2.0.0.8, the certificate generated by the system is not recognized by the VPN configuration, even if it's fully trusted in Keychain Access. As an experiment, I generated my own VPN Server within Keychain Access and tried to import it into the device but no love there as the device responds "Not form our server" (note the typo, as well).
Attempting to use the device with IPSecuritas was equally unsuccessful, as I am asked for a shared secret, which is not a configurable option on the device. The only other option I can think of is to use the Cisco VPN client for OS X, which I am not about to pay for given how the device doesn't appear to work with the OS X Cisco IPSec client.
I wanted to use this device with a client, but as they have Macs in their shop, I just can't reccomend it. A firmware update that enables compatibility (and the ability to load our own certs) would be superb!