Collider problem with terrain generated using terrain toolkit (dev build)

I've got a large terrain object (200km x 200km) generated using the terrain toolkit and I'm working on applying detail central 10km to a section of it.

I need to stop the camera from passing through the terrain, but with a box collider on the camera and the standard terrain collider on the terrain the camera is free to pass through. There does seem to be a slight pause at the point it moves through, but it doesn't stop.

To test collision I tried to drop a sphere on to the terrain, but it seems the terrain collider is the full size of the terrain so it starts up at the highest point and wraps like a box rather than a mesh around the terrain so I can't get the sphere to drop in to that.

I'm using Unity 3.5.0b6 Dev preview so I don't know if there are any bugs in this which might be a problem. I didn't have any problems trying to do this kind of thing last year on an official build.