From what I'm seeing, I can say that Bad Things Happen(tm). When I run my app, I try to initialize the focus to a particular distance; if I then open the camera app, I can often hear the focus snap back to a different level. Moreover, the camera app will die with an "Operation Failed" message. (Possibly this is because my own app is not going into "standby mode" when the camera app starts; I'll have to see if I can find a way to do this.)

In general, though, I'm not seeing a way to tell when someone else is using the camera, or any method of negotiating ownership over the camera hardware.