Rackspace is experiencing intermittent DNS connectivity issues. They are currently working to resolve the issue. This is causing some DNS lookups to fail which results in what looks like a site down error.
Here is a link to the Rackspace status update page:
In the meantime if you are experiencing an issue with connecting to any of the sites wait a few minutes and then try to refresh the page and this should correct the issue.
Rackspace has resolved the issues related to the DNS problems described above. For full information from rackspace please visit their status url, ie https://status.rackspace.com/