I got my G8 GT a couple of weeks ago, and I had my dealer get one from 300 miles away just so I knew I'd get one that had Bluetooth. AFAIK, Bluetooth is only on the 2009.5 models. You can tell if you have a 2009.5 two ways (that I know of.) First off, the last six digits of the VIN starts with a 3. As in 300123. Also, 2009.5's have a cup holder that slides out from the middle cushion in the back seat. It's kind of tricky to find, but it's there.
As for adding Bluetooth, I read somewhere that there is something called a Bluestar module that can be added to earlier G8s, but it plugs in the audio system in place of the OnStar module, so you have to give up OnStar to make it work.