I don't think this has been explicitly covered anywhere on here, so here goes.
If you use the /usr -> share -> tcsh -> examples -> login file (as covered elsewhere), it will manually set the MANPATH variable in the line:
setenv MANPATH "${HOME}/man:/usr/local/share/man:/usr/share/man"
However, since 10.2 (I believe) the MANPATH variable has been deprecated because it is now generated dynamically from the PATH variable with a command also called MANPATH when it is needed. But it will fail if there is a MANPATH variable already set.
Therefore, it is a good idea to remove the line from the aforementioned login file (after making a backup, of course), and remove instances anywhere else of the the MANPATH variable being manually set in this manner.
Of course, this will probably all change with Panther in less than two weeks, but I thought it necessary to say!
[robg adds: I'm not sure I fully understand this one, but it seems worth publication and discussion, if nothing else.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031014053111192