According to Planet5D, Magic Lantern is working with the latest version of 5D Mark III firmware V 1.2.3. There will be significant updates in this version.
Alex over at Magic Lantern announced today:
Not alpha, not beta, nor zeta, just a bleeding edge that happens to work for me. It might work for you or it might not. (note that my standards are fairly high, so what I call bleeding edge, others may call semi-stable or whatever)