Comcast has enabled their DNS Helper service, which denies client hardware the ability to use anything other then their DNS servers. This is an issue for anyone using the ADVANCED DNS/OPEN DNS/Parental Controls with OpenDNS functions. Disabling these functions will restore internet connectivity, however if you want to use these functions, you will need to disable Comcasts DNS_Helper.
Comcast has information on their site for disabling the DNSHelper function
http://dns-opt-out.comcast.net/help-index.phpUPDATE: The link above may have already been disabled. I Found an blog entry on the Comcast Voices site with an updated entry below:
"Update: Domain Helper has been rolled out nationally. More details here, and if you want to opt-out visit https://dns-opt-out.comcast.net (note that link will only work if you're on Comcast's network)."For more information, please visit the page directly:
http://corporate.comcast.com/comcast-voices/domain-helper-service-here-to-help-you