Comcast is rolling out their DNS hijacking service, which redirects requests for invalid domain names to a Yahoo search results page, similar to what Verisign did a few years ago.

At least Comcast allows you to opt out.  Browse to https://dns-opt-out.comcast.net and it will prompt you for an email address (it does not need to be your comcast.net address) and your cable modem's MAC address.  You will receive a confirmation message at that address, so use a real email address.

I'm lazy – my office is upstairs, and my cable modem is in the basement.  I don't want to walk down there, write down the MAC address and walk back up.  Fortunately my cable modem – a Comcast-issued Scientific Atlanta WebSTAR – has a built-in web server running on http://192.168.100.1/.  Before the modem establishes a connection with Comcast you can view all of the options – System, Signal, Status and Log.  Once connected though, Comcast pushes down security restrictions and you can only access the System page.

Fortunately your cable modem's MAC address is listed there.  Copy the MAC address into the Comcast DNS Helper opt-out form, click Submit and wait for the email containing the confirmation link.  Once you confirm the request it will take about 2 days to take effect.