As pointed out by Cyberbob, Inside Mac Games has obtained word from Bungie regarding the Mac Oni demo, and in doing so have dispelled some of the conspiracy rumors floating around the PC Demo release. An excerpt:

I [Matt] know the Mac demo was lagging behind the PC version a bit, but I don't think it was by much; the code changes between the full game's app and the demo app are not major.

The article goes on to discuss actual release date, and may come as a great reassurance to people worried about getting their copy of the Demo as soon as possible.

Update: Matt has posted about the Mac demo in more detail at Oni Central. Among other things, he says:

We caught a problem with OpenGL at the 11th hour. We need to fix this before we GM the Mac CD and release the Mac demo. According to Stefan it's not a big operation and shouldn't take too long, and he will be working with the Oni guys to make this happen.

Other lines in his post suggest that the currently-available PC demo is the final version intended for release, rather than a beta as some have speculated.