As of this morning, much of the Internet is inaccessible to Comcast users in the Philadelphia region.
This post on DSLReports.com shows the extent of the 'issue.' It seems to be a problem in the local DNS servers that direct traffic to the proper places when you type in an address in your web browser.
I called tech support. They have 'escalated the issue,' but have no idea what a DNS server is. Who knows is they can find their ass with both hands and a flashlight.