Have you ever received a copyright infringement notice from your ISP? What should you do about it?

I had a rude awakening today. Though I am at home for the holidays and not with my roommate right now, I woke up to emails from Comcast saying they received a copyright infringement claim against my IP under the Digital Millennium Copyright Act. It turns out that my roommate has been pirating seasons of a TV show through BitTorrent and Comcast knows about it! They of course are threatening to cut off our service. Other than strangling my roommate when I return to our apartment, what should I do? How likely is it that our service will actually be cut off?