Colin Munro has released his MacOS X driver for Xbox 360 controller. Here is more information:

I have created a USB driver which allows you to use the XBox 360 Controller on an OSX machine, including support for the Apple Force Feedback library. I plan to release this as Open Source using the GNU Public License, but intend to tidy the code up a little first - so the initial release for people to try is binary only ;)

The ZIP file contains an installer package, which when run will install the driver, along with a preference pane to test/configure it.

Currently, the following settings need implemented:

I don't think settings are persistent
More settings, e.g. trigger deadzone, button remap/
Force feedback may elongate effects
Force feedback (DirectInput-style interface) emulator may need more work
Preference pane currently only tests rumble motors, by creating a force feedback interface but then just using the Escape functionality
Driver needs to handle more possible errors.