When I had Cox internet (on win2k), I had to change my computer name to the name they assigned it. I think that is how they identify your computer for their service. I didn't run Linux at the time, nor do I understand cable internet enough to explain it any better. Maybe someone else could tell you how to assign the name in Linux. Changing the name was all that was necessary for me to get their service to work in win2k. Good Luck!
Also, check dslreports cox forum (http://www.dslreports.com/forum/cable,ath), there are some pretty nice folks over there as well.