4/2/08 - Added php.ini settings to use the proper MySQL socket at /tmp/mysql.sock.
3/23/08 - After going through these instructions on a new Leopard system, I made a few minor updates. The steps should now work for Leopard as well as Tiger.
While options abound, MacPorts may be the easiest option to configure a local web development environment on your Mac. I'll mention a few of the other options and then share the steps I've used to install Apache 2 and PHP 5 with MacPorts.