Remember this is part of a bigger proposal, that if you allow your Comcast connection to be used by others, you in turn will get to use any Comcast access point.
Not quite. I had a chance to see the internal Comcast documentation on this once. You won't have a choice, if you have one of their wireless gateways, it will eventually be turned on for you and you can't turn it off. However, the XFinity wifi network is completely separate from your network, so unless someone finds a flaw in the firmware it should be secure. And, your network gets higher priority, and any data usage by a wifi customer doesn't count against the download speed you pay for (if you pay for 30mb down you can get the full 30mb down while another customer uses the wifi).
If you're not careful, you're going to catch something.