I've been having this problem for months now. Comcast will just drop all HTTP connections to Google's servers. I can resolve DNS just fine (today's numbers are 72.14.253.104, 72.14.253.103, 72.14.253.147, 72.14.253.99 - these change based on your geographical location), but HTTP connections to those IPs will get immediately dropped through Comcast. If I SSH Proxy to my server in a local data center, I can connect to those IPs just fine.
I spoke with Comcast Tier 1 tech support, the kind woman I spoke to for an hour couldn't figure it out. We reset my modem, firewall and computers multiple times without luck. Supposedly this has been escalated and I'll be getting a call back from a higher-up, we'll see.
At what point does blocking a website mean that they are no longer providing internet service? What if the only site I could browse was www.comcast.net? This really stinks of the larger net neutrality issue, and problems like this will most likely continue unabated until legislation is passed. Comcast has no reason to stop screwing their customers, since they have such little competition. The only other internet service I can get is Verizon DSL, but they don't provide the same speed service. If we had more options and more competition trouble like this would be short lived.