We've been using valgrind on the mac for several months now
on the Chromium project. We run some of our unit tests
constantly under Valgrind, and it has caught quite a few
leaks and other problems. I highly recommend it.
http://groups.google.com/group/vim_mac/msg/2bfd871b82c9b3b1 is a
walkthrough
showing exactly how to build and use it to track down a bug in mac
vim.
(Though that was done before the merge to trunk, so the download line
is a bit dated.)
Run, don't walk, to http://valgrind.org, check it out from the trunk,
and give it a shot.