VPN tunnel between a PIX on a private address being natted, and a 2600 with a public address? It seems to me that it won't work because you need a peer address configured on the 2600. Or is there a different way to do it?
If you are doing a One to One Static NAT translation for the PIX firewall, then this should work without a problem, but if the device in front of the PIX is doing PAT then it would be a problem to do...