<p>Does anyone know how to setup <a href="http://www.selenic.com/mercurial/wiki/index.cgi/Mercurial" rel="noreferrer">Mercurial</a> to use <a href="http://www.perforce.com/perforce/products/merge.html" rel="noreferrer">p4merge</a> as the merge/diff tool on OS X 10.5?</p>

I'm guessing there's a CLI tool for p4merge (which I know nothing about).
I wrote a blog post about using Changes.app, and some other GUI tools with Mercurial:
Using Mercurial with GUI Tools.
Basically, you need to know the calling expectations of the CLI tool that loads up the diff tool.  IE, how to make it load data from a particular file, and how to make it wait for exit.  There should be enough info on the post to give you some ideas.