Well, if it weren't I would connect everything I own to that guest access point and bypass my cap completely. Or perhaps the speed of that connection is very slow. But then it wouldn't be much use to other customers either.
When you connect to an 'xfinitywifi' SSID, you have to authenticate with your comcast credentials. The usage is then tracked as yours, not whose-ever hotspot you hit.
This works and you can do it today. You actually can get faster than your current service plan (assuming you aren't at the top-tier already) if you have a linux router you can set up for dual-wan.
Last I played with it, I could get an additional 35-40mbps or so out of a typical 100/25 comcast connection in my area.
