Perhaps you've already had luck activating your comcast connection, but I'll share my comcast experience.
My digital cable and hi-speed internet were installed yesterday and the installer left an installation CD with my wife. Of course, the installation CD is for Windows only. (maybe even Mac).
I configured my Mandrake 10 laptop to use DHCP to obtain an IP address and attached the ethernet cable to the cable modem.
The laptop obtained a dynamic IP fine, DNS worked, but I could not connect anywhere with telnet or a browser.
Judging from the sparse documentation with the CD, I had to complete some sort of registration process. But where to register?
I phoned Comcast support and talked to a fine fellow who was not very familiar with Linux. He told me Comcast only supports Windows, and I should phone my linux support to help me connect!! And he didn't know what DHCP was.
Finally, I presented him with this scenario: if someone with a Windows computer had broken their installation CD, how could they connect? Eureka, a question that he was familiar with and had a script to guide him down the path.
Turns out http://sas.r2.attbi.com
is the magic location for completing the registration. I needed my account number (subscriber #) and registration number (beside my address) which I obtained from my installation receipt. I completed the registration, reset the modem and ba-boom, connectivity success.
Not sure if that's what you're looking for. hope it helps you (or someone else).