Found this today, more rumors about the Z4M and a COUPE also:
BMW Z4 M Roadster
BMW is preparing a hot M version of the Z4: this prototype snapped here has wider wheels, subtle aerodynamic enhancements and twin pairs of tailpipes, all hinting at the roadster's performance potential. It's likely to be fitted with a version of the M3's 3.2-litre straight-six engine, which delivers 343bhpin the M3, and six-speed sequential transmission. We've not seen pictures of a coupe version yet, but one is said to be in the pipeline to follow on later after the launch of the M Roadster.