I was successfully using the checklink service this afternoon. Around 18:00 EDT I started getting the following errors. I was able to reproduce the problem with other websites. I also did a telnet on port 80 from a remote server to eliminate problems with my browser or ISP connection.
PS. Very useful service, by the way!
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /checklink/checklink.
Reason: Error reading from remote server
Apache/2 Server at validator.w3.org Port 80