From The Internet Storm Center
"We received a couple of e-mails over the weekend asking us why this vulnerability was significant. Most public DNS servers should not be listening on the RPC ports, after all. Indeed, networks obliging to basic secure perimeter design would only allow port 53 UDP/TCP to the authorative DNS servers, and definitely not the additional RPC ports required for exploitation."
SANS Internet Storm Center; Cooperative Network Security Community - Internet Security - isc