Artur Ejsmont has a recent post to his blog showing how to get a MongoDB PECL extension to compile in a 32bit OSX environment (Snow Leopard).

Here is a quick step by step guide on how to get mongodb and PHP5.2 mongo pecl extension going on your MacOSX in 32bit mode! NOTE: 32 bit mongodb binaries have 2GB address space limit so you wont be able to process too much on your laptop. You will still be able to code and connect to remote instances just fine.

His process includes five steps - well, eight if you count the optional "install MongoDB" ones too - complete with the commands you'll need to get things compiled, ready for copy and paste. You can find the MongoDB PECL package here.