Found these posts by Scott while digging around on **************.
Recently there was mention that they did not "Bite the bullet" when deciding which name to use, however that doesn't mean much unless you know what the "bullet" is.
The second link hopefully gives the clue:
http://www.**************.com/1-seri...tml#post457180
http://www.**************.com/1-seri...tml#post455245
Unless I am reading this wrong, I can only conclude that M1 (on its own) will not be used.
I suppose this does not rule out some derivation of M1 such as M1 Coupe.