Typically I develop my websites on trunk, then merge changes to a testing branch where they are put on a \'beta\' website, and then finally they are merged onto a live branc
We have it setup much like Toby. A series of config files for each developer, that has the Facebook APP Id info (a different app for each developer), separate pages where the app is hosted, and git ignores the config files. We're LAMP with Code Igniter, and it's similar to Rails in that we can set the environment in 1 file, which points to the config with the Facebook constants.
Branching out into Selenium, using unit tests for model-testing.