Probably it will just depend on the manufacturer and their support with voice control. Maybe not immediately but when they get their hands on the 3GS they will be able to develop a way to get it to work. And who knows if and when Apple may release a new bluetooth headset made to work with voice control and have great stereo bluetooth sound.
Currently you have to hold down the home button to get the voice activation to occur so it's unlikely that current bluetooth devices will be able to do it, but who knows. Apple may have included it in the API to activate.