They can actually look at the User Agent string that the browser is passing and block it that way if they wanted. So it's quite possible that Comcast does have a badly configured proxy. If he were being blocked by a software firewall, it probably wouldn't be blocking IETab too.
You can try changing your User Agent in the registry. That might let you get around Comcast's block. You might also want to call Comcast and see what the deal is.