Nico,
From what I've read in Kong's post in the SIHH forum, I am pretty sure that the "composite" case is a case-hardened/surface finish similar in concept to PVD, CVD, etc. The base material for the case would appear to be Aluminium alloy. Not that it matters as it's a great looking watch.
See below for a cut'n'paste of reply to Kong's post:
"...sounds like the Microplasmic process - a new(ish) process whereby AC current is applied to the aluminium - in this case, watchcase - in a special electrolyte bath, the high voltage current causes plasma to form at the surface of the aluminium to form the hard film. Very cool process.
Is the watch particularly light? It is particularly handsome."