Rackspace uses anycast network for its DNS servers

From the DNS FAQ on the Rackspace Knowladge site cloud_dns_faq we can read out that:

What type of DNS network does Rackspace use?


Rackspace leverages a globally distributed anycast network. Currently we have DNS servers located in Texas, Virginia, Chicago, Hong Kong, and London. Using anycast, we broadcast the IP addresses for ns.rackspace.com and ns2.rackspace.com from each location. All DNS queries will generally go to the geographically closest nameservers, giving you faster results no matter where your queries originate. Additionally, all of our DNS servers are monitored 24/7 and if an entire datacenter were to fail, or even if all of the DNS servers within a specific datacenter were to fail, the DNS queries will automatically start going to the next best location.



View Rackspace anycast DNS locations in a larger map

References
http://www.rackspace.com/knowledge_center/cloud_dns_faq