I don't have Comcast, I have Time Warner, but cable modems pretty much all work the same way...
Your cable modem should have an ethernet port. Just connect an ethernet cable from the cable modem to your iBook. Also, make sure your iBook is configured to use DHCP to get an IP address automatically from your cable modem. To verify this, go to System Preferences. Then click on the "Network" icon. Then select "Built-in Ethernet" from the dropdown menu. Then make sure you select the option to get an IP address automatically using DHCP.
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.
On an interesting side note, I got home last night and lo and behold, my comcast signal is not responding. I even rebooted the modem, but no dice. It popped back on after a while though. I nothice when it doesthis that it has dropped its mask adress (usaully 255.xxx.xxx.0 becomes 255.xxx.x.0) anyone know why that might be?
As far as the loss of connection , I have the same thing happen to me about once every two months. The subnet mask changes because the DHCP server has not given out an IP address and the computer just goes to a random (or maybe not so random) but obviously non functional mask. Its Comcast not your computer I can tell you that for sure.
Recently in my area they have started changing over and ubgrading the cable modems. I have seen a few of the new ones and I remebered seeing those exact ones in an article about cable ISPs trying to start to charge a per mb fee - like the old days of dialup . Dont get too worried about it yet though , there is alot of opposition to this on a small grassroots front as well and lawyer / lagal front. Ugh - I ramble..anyway I would image the outages might be due to upgrades and changes in thier system.