When I get my Mac Pro some time in the near future, it will do what my iMac has done, but more efficiently.
I love gaming, so I will have bootcamp on there with Windows 7 for that (though Mac gaming options are improving).  It's really the only good mac for gaming since gaming requires upgradeability.  I don't like calling myself an avid gamer, but I spend a good deal of time playing them.  I just don't really enjoy first person shooters and war games, like, at all.  I'm more of a Sims 3/SimCity/Civ IV/V guy.
It will also serve as a media hub for my terabytes of music/movies/TV shows.
I recently purchased a 32" HDTV so I'll probably get the AppleTV as well.
Basically, it will be the nerve center of my home.