I just got a new Comcast high-speed cable account today. It was a bit of a hassle to get the setup utility to run properly on OSX, but it did eventually. Then I had a working connection from laptop to cable modem to the world.
I wanted to use my Airport Express as a wireless router; this went less smoothly. I fiddled with this setting and that, but could not get the Airport Express to get a DHCP lease. This discussion, however, solved my problem immediately: the trick is that the cable modem remembers the MAC address of the computer you set up the account with. To fix the problem, just power-cycle the modem while the Airport Express is plugged in to the LAN port.
