yes - magnesium is prone to high temps. - when the car catches fire it melts like ice cream - but i guess it doesnt matter... does it Nick?
Magnesium wheel is just the wheel casted from different alloy - containing a lot of magnesium. There are some problems with rigidity of magnesium structutres - Magnesium is softer then Aluminium, but manufacturers surely sort these problems out somehow their own (highly confidential) way. - all WRC factory teams (and some privs too as I belive) rally cars use magnesium wheels - speedline, OZ(=enkei on Mitsu)
As I know there are only a couple of makes that made magnesium wheels on bigger scale and available to private customers - that is Speedline, OZ(=enkei), ATS (which no longer does), and in m.sport only BBS.
in addition many less known or popular companies like Volk racing and others.
Major problem for manufacturers is painting magnesium wheel, as normal powder coatings arent really ok with magnesium alloy. Painting magnesium is more complicated.
Magnesium wheels RUST! very easily and quickly - which adds to painting problem.
I guess thats all - at least all that happend to come up to my mind