Hello Mark,
I have spent some time with the Cisco SE on this one. I have just scoped up pair of 3800's with VPN-SSL3 AIM cards to be my key servers.
I asked the same question as you, but there seems to be no real data on this one apart from the in the data sheet for GET VPN.
GET VPN Key Server
Cisco 1841, 2800 Series and 3800 Series,
AIM-VPN/SSL-1, AIM-VPN/SSL-2, AIM-VPN/SSL-3* (Cisco ISR)
Cisco 7200 Series and 7301 routers
Cisco VAM2+, Cisco VSA** (Cisco 7200 Series and 7301 routers)
Hope this helps