This site has been using PostgreSQL as a back end for WordPress since the beginning. This is possible with a nice little plugin PG4WP that will translate queries on the fly into something that PostgreSQL can understand.
I’m a big fan of PostgreSQL and use it on almost every project I do. There is something about setting up a MySQL instance and managing it that bothers me, so this plugin makes my day.
I have pushed a copy of the plugin to GitHub so that I can keep track of tweaks I make to it as I find compatibility problems. I am now running WordPress 3.0 and am monitoring my logs for database errors.