There is a useful command-line utility known as 'dnstracer' written by Edwin Groothuis. dnstracer is available (in source code form, requiring compiling) from his web site.
It shows you which servers are responding to your DNS queries: those that have cached info and those which have the authoritative info. It seems like this would be very useful for debugging DNS problems.
I had no difficulty at all in compiling this. Following the instructions in the README, I ran ./configure and then make. I already had the Developer Tools from Apple, which are required to compile source code.
Mac OS X Hints
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