Got an email from Comcast - is this something I should worry about?

If you are downloading copyrighted material illegally, stop doing it. If you haven't downloaded anything illegally, you can argue with Comcast—the mere fact that a copyright holder asked about your IP address doesn't necessarily prove that you've done anything wrong, as there are many variables. If you haven't downloaded anything illegally and Comcast terminates your service, you can sue both Comcast and/or the copyright holder. However, make SURE that you haven't actually downloaded illegally before you argue or sue. If you are infringing copyrights, your best course of action is to stop infringing and keep a low profile, and maybe you'll avoid any further trouble.