My DS had the same problem as well. The camera *sometimes* refocus when the shutter-release is fully pressed. I sent my camera back (still under warranty) for a repair and now it works the way it should.
I think this is not an uncommon problem. Another user reported that he got the same phenomen with his *ist film camera.
An easy way to test the shutter-release button is to take a picture first, then do not fully release the shutter button, and try to press the shutter button again. Try repeating the process (press, half release, press, half release....), the camera should never take a picture as long as you don't fully release the button. Of course you need to set the camera in AF, AFS and single shot mode.