Can't 'find' router address - new Imac + cable modem

This is a new out of the box 20' Imac. When I connect the enternet cable from the cable modem, i can get a dhcp lease/address, but can not get a router address and therefore can't connect to the internet.

If i take this same cable and plug it into another older imac, refresh the lease, and voila, the router address shows up and i can connect. This older imac doesnt use any proprietary internet connection software at all, just connects thru the standard DHCP ethernet connection.

Rebooting the modem, the computer, doesnt produce any new results.