Phaux
Magnesium might be used in any number of casting situations where weight is a primary factor in design. In some cases a weight reduction of as much as 30% can be achieved by using magnesium in place of aluminum castings.
Aerospace, Automotive, Motorcycles... Lot's of applications
Here is a small bit of data reporting on Alum vs Mag on Ford Contours in the 1990s.. There are some discussions about benefits and drawbacks but not much on actual material performance
http://www.p2pays.org/ref/03/02602.pdf