Also, if it's a MAC address thing many routers has a "mac address clone" feature that will copy your computer MAC address into the router's MAC address so cable internet providers won't complain. I think that's available in your model linksys, it's available in my linksys wrt54gs.
Check the status in the router to see if you get an external IP address too. Assuming you can talk to your router, which you should be able to do regardless of the WAN side.
Bartender to Rene Descartes "have another beer?" Descartes: "I think not" and he vanished.