Their DNS is broken. They have a CNAME at the root of their zone, which is forbidden. If you're using the "Unbound" DNS resolver (I am), and it's in DNSSEC validation mode, then it will SERVFAIL when it comes across broken DNS like this (even if the zone isn't using DNSSEC).
I don't know about other DNS resolvers, but IIRC didn't Comcast switch to using resolvers which support DNSSEC for their customers recently?
[edit] - Can you see http://imgur.com/ on Comcast? That (well known) site has the same problem. I tried to alert them to this fact a few weeks ago but never heard back. I'd be surprised if Comcast users can't access that site.