I'd called a couple of days ago to make sure of the date and time - since that little detail was not included on the confirmation email from Comcast. I spoke to the telerep, made one or two minor changes on my account, and then confirmed the date/time again with the rep before hanging up. So at that point, all was seemingly well.
Apparently what went wrong was that when the rep changed one of the preferences in my account, their system for some reason defaulted to the "self-install" option and cancelled my tech appointment without anybody realizing it. This would be technically an error on the part of the telerep, I guess, but from multiple interactions with Comcast it sounds like their CRM system is not the easiest thing to use.
So I think this is another one of those cases where there isn't any one individual to blame - nobody was being a "bad employee" or "not doing their job" - it's the fault of a clunky and poorly designed system. Ideally, the folks whose job it is to answer the phones should be provided with efficient, user-friendly software tools to help them do their jobs; at a minimum, they should be adequately trained so that they understand every ramification of changing this or that customer preference.
Anyway. I am still without Comcast service for the time being; fortunately my BlackBerry does a kick-ass job as a wi-fi hotspot, so I'm able to type this post.
So I'm still waiting to hear back from Comcast. But the good news is, the move is almost over and done with.