<p>How can I get PDO to work on my mac (os x 10.5)? I'm using the built in php and php in Zend/Eclipse. Can't seem to find useful drivers for it at all.</p>

I'm not sure this will help with the PDO drivers specifically, but you might look into
BitNami's MAPPStack
.
I had a ton of trouble with Postgres, PHP, and Apache on my Mac, some of it having to do with 64- vs 32-bit versions of some or all of them.  So far, the BitNami MAPPStack install is working nicely in general.  Maybe it will help with your PDO issues as well.