Our servers use a distributed DNS system with primary name servers on enterprise-level computer systems in geographically separate data centers. It is as reliable a system as we have seen on any hosting company servers.
Many hosting companies run two name servers on the same server as your website. This is insecure, and unreliable because if the web server goes down the DNS needs to propagate out when the system comes back online. Our DNS system continues to work regardless of any issues with your website. Some hosting companies will put two name servers on different physical servers in the same data center. This is not sufficient because if they have servers in more than one data center and the line to the first location goes down, all the sites that they host will disappear from the Internet.
In the unlikely event that our master DNS server fails we have a backup DNS server running in parallel. Some of our competitors think this system is overkill, but you should take this as an indication of how seriously we treat the business of hosting your website.