One of the other criteria, at least in the past, was whether there was a low-pass filter on the line. They don't use them in my area, anymore, I'm pretty sure, but I wonder if they use them anywhere anymore, or more specifically, whether there could be some still installed on some outlets in some subscribers' homes, interfering with the ability to put the cable modem on that outlet. (I remember being told specifically that the line going up to the modem couldn't have that filter on it, so when they put them in they put them on two of the three lines coming out of the first splitter, in my basement, and then on the splitter by my cable modem, on the "other" output of that splitter.)
Anyway, my question is whether or not low-pass filters are still in use anywhere in the company.
With regard to the OP, if this would be an issue, you can see the filter installed, typically as the first splitter coming into your home. They look like this:
http://www.summitsource.com/product_...pg&w=150&h=150
Mine generally had a blue stripe around them, though I've seen them with a gold-colored stripe. If you do have filters you can see, then we need to find out which lines go to which outlets -- specifically which line goes to the outlet you plan to use (exclusively, from what you've said) for your cable modem. That being the case, you could remove the filter from that line, and save yourself the cost of the Comcast service call.
For that matter, in reviewing where Comcast service enters your home, you might find that the lines to the outlets are just disconnected. You could reconnect them, if you had a high-enough quality splitter. That's the first thing that a Comcast tech would try. However, not any-old splitter that you can pick up at the Radio Shack will do.