Spec'd my M6 coupe today here in the UK.
New BMW bulletin confirmed that 2013 M6 coupes will come equipped with a rear trunk spoiler not seen on any of the pictures or motor show cars to date. It's been added for enhanced aerodynamics. Bulletin included generated pictures of rear spoiler. Here are some real life photos thanks to DiavelM3.
Should also make for an even more aggressive looking M6.
Admin Edit:
Added gallery with new spoiler:
Compare to M6 without trunk spoiler: