At first I would say no, because a Z4 3.0i fully loaded already comes to about $50,000 MSRP. How many people are going to be willing to pay another $5K-$10K for an M version of a tiny little Z4?
But now that Mercedes has rolled out their new SLK55 AMG (0-60 in 4.9 seconds vs the Z4's 5.9 seconds) that comes to like $68K fully loaded, I think it's only a matter of time...