I think the short answer is no. I just tried to setup my new Motorola SB6121 to replace a Comcast-issued gateway (Arris TG862) and I could not get it activated. I called 1-800-COMCAST and they said I have to speak to the Activation Dept. which is only open Monday - Saturday from 8am-5pm ET.
When an unregistered modem is connected, after it finishes any firmware updates, SHOULD connect you to the 'walled garden'. This is a restricted area that lets you register the modem. Normally, you should try to connect the computer directly to the modem if possible.
The problem is that this doesn't always work. When I recently replaced my modem, I still had to call in as I didn't get anything at all.