Check out MWA, which has been updated with some bits about Myth III and MacWorld. In one forum post, Andrew Meggs said this:

I'm surprised to hear there was anything at all Myth 3-related at MacWorld on the first day. Tomorrow (er...I guess it's today now, but anyway, the second day) I urge everyone to stop off at ATI and/or MacSoft. There was no official plan to show the game, but the dev team got all sad about it and sent something out this afternoon.

A lot of people have raised concern over the fact that reports keep stating that Myth III will ship with only five unique netmaps, but Meggs had this to say:

Actually, there are more than five multiplayer maps. The exact number that make the cut will depend on playtesting, but the 'five' figure that keeps getting tossed around is one of those 'underpromise and overdeliver' things.

He also says that the Myrk model has been reinterpreted for the MacWorld demo.