MongoDB was mentioned a few times at tek and I said that I wanted to have a look at. Travis' article, MongoDB: A first look, came out a few days ago and piqued my interest further. Then Matthew sent me some source code that requires it. The stage was set for getting MongoDB working on my Mac.
Using the homebrew utility he was able to easily install MongoDB itself and setting up PHP support for it was as simple as calling the "pecl" command line to grab and build the shared object. Drop it into your php.ini file and you're all set to go.