Are you specifically looking for a built in one? Otherwise, you can run a cable from your modem into your wireless router (Comcast gives wireless-N routers for only the cost of shipping, or you could pick one up at a Comcast store), and use your router as an internet hub. True, it's two separate devices, which you may not want, but it gets the job done cheaper.
I can't think of any, nor have I seen a router, with a built in modem. If I have, it had only one Ethernet output, which is somewhat pathetic.