Wouldn't Apple be able to sell more computers by offering a mid-tower design for about $1200? *snip* I would love to see a Mac mid-tower that looks a lot like Dell's Inspiron. The fact that most PC desktops are mid-towers and Apple doesn't make one STILL kinda surprises me, especially when they have such a jones for the MacBook Air (twice the price, half the guts!).
By mid-tower are you talking about a machine 15" tall and has to sit on a separate shelf or cupboard in order to leave desktop space open for other purposes? To me that's a full tower, not a mid-.
However, maybe some enlarged version of the Mac Mini would serve... say, 6 to 8 inches tall, about the same deep and about 6 inches wide... similar, perhaps, to the Shuttle and some other compact PCs I've seen and built. Give it all the power of the iMac while making it possible to upgrade video (as can be done in the Pro,) double or even quadruple the amount of on-board RAM, etc. Now that would be nice.