adephue - 2:40 am on Aug 11, 2010 (gmt 0)
You have to pay close attention to name servers offered by hosting companies. In some cases it is one name server that has two IP addresses.
I like to utilize either third party DNS services or host my own on separate physical servers. I wrote a primer/proof-of-concept that outlines failover name servers for reliability and uptime [blog.dephuedatadesign.com].