Apple's latest Bluetooth update (v1.5) and Firmware upgrade (v.1.0.2) allow you to connect your Bluetooth headset to your Mac. You must install both of these updates for it to work, and you must use the Bluetooth Setup Assistant to pair the headset otherwise it will not appear as an available sound input/output in System Preferences, or iChat AV's preferences.
Download and install the firmware updater. It takes several minutes to upgrade the firmware so be patient. The update is compatible with Apple's internal bluetooth modules and the later model DLink usb modules. Install the Bluetooth 1.5 update which should appear in Software Update or can be downloaded here.
Go to System Preferences, Bluetooth and click on Set Up New Device. Select "Headset" from the list of devices to search for. If Headset does not appear in the list, you have not updated the Bluetooth firmware. Your headset should appear in the devices found list - select it and continue the process to pair it. Once paired, your headset should appear as an option in System Preferences, Sound, In and Out. If you only wish to use the headset with iChat AV, select the headset in iChat AV's own preferences.
Those of you hoping to use your headsets with Apple's speech recognition are out of luck - Apple mentions this in the notes to the Bluetooth update.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040208115122220