<p>Is there a tool, method or setting in the standard VBA Editor to warn about variables that have been <code>Dim</code>'med, but aren't being used?</p>

MZ-Tools
will search through your code and tell you what is not being used. The version for VBA can be found
here
.
The specific feature in MZ-Tools that performs what you asking about is
Review Source Code
:
The Review Source Code feature allows
  you to review the source code to
  detect some unused declarations
  (constants, variables, parameters,
  procedures, etc.).