bluetooth was actually only equiped to the late 09 makes, even the first batch of 2009 G8s didn't have bluetooth the OnStar system on the OnStar phone(hence the "bluetooth button" on call G8s).
I am not sure if you can get it flashed into it or not but I would definatly be interested in finding that out becuase my 2009 G8 does not have bluetooth so if anyone has had any luck please let us know
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.