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:
https://status.rackspace.com/
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.
Updated 10:56am
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/