According to the link below, you may have spotted the new 6-series prototype convertibile.
Different license plate though. Look at "Bob"s post. He lists a few more sightings including a BMW with a plate M DJ 2804. As far as I know, "M DJ" is just a normal plate and not something reserved for BMW.
Last edited by ViaCorsa; 11-03-2010 at 12:25 PM.