bobindashadows has posted over at Halo Mac Development that after a couple of weeks of work, he has successfully converted a map created for Halo CE, in this case the desert island map by ZexGX to work with Halo for the Mac.
Five screenshots accompany the story, showing ZexGX's map clearly running in Aqua windows.
Unfortunately, bobindashadows says that quite a bit of hand-editing work is required to do this conversion, so it looks like there will not be anything like a drag-and-drop converter anytime soon. In the absence of such an app, he'll only be converting the particular maps he thinks of as being worth it.
Thanks for the heads-up on this story to Louis Wu at HBO.