Thanks for the speedy reply.

I saw a townhouse (built in 2004) with an exposed garage ceiling, with just insulation. It has a sprinkler system installed on the ceiling, but that's it. I wasn't sure if this was a valid method of fire separation or not. I think they did it because some of the a/c ducting runs across the garage ceiling, which would then drastically reduce the headroom if a drywall ceiling were installed.