I think this is a pretty good idea, and something I have given a lot of thought to. The problem is really bigger than simple privacy though. If you are going to take the time to build an open router, it would be wise to make it have significant broadcast range to try and build in mesh network support.

If people could pay a flat hardware fee and connect directly to the internet in a secure way, Comcast would have significantly less bargaining power and the customer would have significantly more privacy.