They can already sell your info without this new service. See earlier stories about ISPs doing so.
But this new service makes me wonder about new exploitation. According to the page, users have to download and install some Comcast software. I'm guessing it is closed-source, with a "licence agreement" that lets Comcast do whatever it wants with little or no protection for the users, and has to be installed with root privileges on the computer. Can anyone who has tried it confirm?
So probably Comcast is going to have free reign to do whatever they want with users' data. Do they at least promise not to sell it to advertisers?