Today Apple released the Bluetooth Software 1.5 Update, which, among other things, allows you to use Bluetooth headsets with your Mac.
There's a catch, you see. That catch is that in order for your Bluetooth headset to be recognized by the Bluetooth Setup Assistant, you'll likely have to also install the Bluetooth Firmware Updater 1.0.2. Without this firmware update you'll be able to pair the headset from within the Bluetooth system preference, but no action results in the all-important Input portion of the Sound system preference or Apple's Audio MIDI Setup utility. Once you've installed the firmware update, a new "Headset" entry appear in the Setup Assistant and you can pair the Mac with the headset.
Those with new Macs aren't out of the woods. Even though the firmware update was last updated in October of 2003, my three-week-old 15" AlBook required it in order for the Bluetooth Setup Assistant to see my Jabra BT200 headset.
Note that you shouldn't expect stellar results from your headset. My Jabra headset records only at 8KHz -- voice quality -- and I had to push GarageBand's input all the way up to get even mediocre gain. Also, Apple tells us that speech recognition doesn't currently work with Bluetooth headsets.