TeamXbox has an interview with XNA developer Scott Henson at Microsoft about what exactly XNA is going to do for the average gamer. One scenario he imagines is where users on Windows PCs play one kind of role in the same game as console players fulfilling a different role:

So from a gamer point of view, I could take on the role of the commander. I could use the hi-res screen of my PC and act as a general leading and directing my troops. The guys/girls that you're playing with are on a console in the battle fighting it out. So one person is seeing the results, commanding from one point of view on a PC and then there are others having that visceral, down in the trenches experience on Xbox. So, for a gamer, I just think this causes an explosion of possibilities.

So... will there be a Mac version? Just kidding.