DHCP is key with the comcast setup, so make sure its set for sure. If you have the surfboard modem (my comcast modem) make a visual check for incomming signal. It takes them a couple of hours to turn on service, so you may not be ready yet.
Also, if you have a house wired for cable, with different outlets in different rooms, you are only going to get internet signal through the main outlet. I think you can call them and have them switch the outlet its comming from, but you'd have to check. (personally I just got a hundred feet of cable and a wall drill, but I like doing crap like that.)
Hope that helps. Comcast supports X, so they should help you on the phone, but both my Pismo powerbook and my tower get the signal imediately without setup, so double check the settings.