I bought the Cisco DCP3939 and tried to provision it. Comcast told me to take it to their service center because it had been stolen. I contacted Comcast again today and they confirmed the gateway had been stolen.
Had I retuned it they would have charged me 20% to 30% (they surely would not have) to check the modem.
Also they charged me $10.00 for 2 day delivery - which they do anyway.