We are experiencing and have had a number of clients (who are also Comcast customers) note that there appear to be some issues on the Comcast backbone. The symptoms that we are seeing thus far are problems accessing very specific networks and hosts, most notably Google.com (Gmail appears to be up but several other Google services like Google Maps, YouTube, etc. are not).
If you are having trouble accessing some sites on the Internet and you are a Comcast subscriber, please let us know so that we can pin down where the issue is.