I searched the Apple site with little success. The iMac page talks about Bluetooth but makes it apparent that feature is an add-on. How do you add Bluetooth? Use the user slot where AirPort would go or external USB add-on device? I'd prefer an internal solution versus chewing up a USB port, though.
Don't think it's possible to add internal bluetooth without voiding the warranty on the iMac. I would just go out and buy a USB bluetooth adapter. D-link makes a good one that I think is the most recommended for the Mac poatform. - reaper
...and I found the answer: "Note: If your computer did not come with a Bluetooth module already installed, you need to connect a USB Bluetooth module to your computer in order to use Bluetooth."