I'm still waiting on my Terrain, but from messing around with the model in the showroom (SLT1, no nav), there doesn't seem to be any stereo bluetooth support. I paired an iPhone 3G, and wasn't able to get it to play via. bluetooth at all. The phone itself said the stereo BT wasn't supported on the connected. I know Sync has it, and it works well - I had a chance to play around with a Flex at a Microsoft show (Sync is admittedly heads and heels above any other infotainment system).
This is one of my personal small gripes with GM (and most other car manufacturers). Adding something like stereo bluetooth would cost close to nothing, as it's ultimately just a matter of the bluetooth chip inside the stereo needing to support it. We're talking the difference between a $12 chip and a $14 chip, and even then most BT chipsets fully support all BT profiles now (like stereo BT). It's silly that it's not supported, but oh well. Except for Sync, most cars' BT implementations leave a bit to be desired. Even BMW is extremely lackluster in the BT/iPhone support department.