I'm using my tomato behind an other router (ADSL box).
My tomato is connecting through LAN port, so my WAN IP is local (192.168.1.51)
I'm using my first router for a "classic" connection, and my tomato for VPN tunneling. (It works great with VPNtunnel.se)
The external IP address checker doesn't work ! (= "Error obtaining IP address")
Here is a screen :