There is a writeup of his techniques in B+W magazine. He exposes once for the stars and once for the landscape. They are then composited in photoshop. I have to admit that this is one area where I agree with digital manipulation, when digital techniques are used to overcome some limitation in 'straight' photography. These images couldn't happen without digital manipulation, but every attempt was made not to alter the 'spirit' of the scene. Here is another one I like done in a similar way....
http://www.photo.net/photodb/photo.tcl?photo_id=432024