A post in the Yahoo group mssmallbiz asked an interesting question:  when DNS Forwarders are configured under SBS 2003, is there a way to get the secondary nameserver to automatically take over when the primary is unavailable?

Don't Use Forwarders

The answer was simple:  don't use forwarders. Go to Start > Administrative Tools > DNS, go into the Properties for the server, go to the Forwarders tab, and click Remove until they are gone.

Do Use Root Hints 

Then make sure the root hints are up to date. The poster suggested using Windows Update. I didn't see root hints there, but I found a simple way to refresh them:  in the DNS Properties, go to the Root Hints tab, click on Copy from Server, and enter 4.2.2.2 as the IP address to copy the hints from.

Posters seemed to think that relying solely on root hints is actually faster for resolving external DNS queries.

The instructions work for Windows Server 2003 as well as SBS 2003!

Update:  copying root hints from 4.2.2.2 doens't work. See this post for details and a solution.