I placed a MS Windows 2000 Server configured as a PPTP Server behind a Cisco2650 (IOS 12.3) router doing static NAT translation of the PPTP port and IP port. The Cisco was also doing overload NAT translations of the internal ip address range. The 2000 RRAS Server was obtaining int ip addresses from an int DHCP server.
In the present Cisco IOS 12.3. incarnation this does not work. The cisco will not handle the underlying PPP protocol with overlaod NAT, but will work if static translations are in place.
The PPP protocol does not expose ports that are needed for the overload NAT to keep track of the traffic.
Nonetheless a NetGear FVS318 router does handle this by using PPP session ids (or at least I think that is what they are using) to do the differentiation.