Two vendors stand out as having particularly poor DNS infrastructure. Caldera maintains by far the worst, with only two DNS servers hosted at the same site. In fact, this is the site that hosts most of their servers, email, FTP and so on. Essentially, they have a network link to their offices with all their infrastructure based there. If someone were to flood a router on that link, they could likely take out Caldera entirely DNS, email, secondary email, FTP, etc.
Mandrake is another vendor with a poor DNS infrastructure.
While not nearly as bad as Caldera's, Mandrake's is far from perfect. Thus an attack on the DNS servers themselves is possible. And these companies are not alone. However, as you can see from the table, it is possible to do it correctly. This is not a network task that should be put off.