Apple is not going to release a mid-tower. Period.
Number one, it would probably cut in to Mac Pro sales (seriously). Number two most people who buy computers don't WANT to open their computers and mess with them. Sure, a lot of people on here want a mid-tower, but that is a huge minority compared to the total market. I think people on message boards sometimes forget that they only make up a few thousand people. There are lots of reasons why a "mid-tower" does not make sense.
The argument is always "it would be convenient so I could add stuff to it". You guys are not understanding the whole point of Macs..."built-in, not bolted on" or "it just works".
The Mac Pro is the machine they make for people who want to interchange and add stuff to. I am sorry that it is out of most people's price range, but it goes against Apple's philosophy to make a cheaper machine that is geared towards minor upgrades.
It would seriously tarnish the reputation of Macs if people had a mid tower that they were putting random stuff they bought at Wal-Mart in it and wondering why it doesn't work. People forking over the money for a Mac Pro are probably more educated about the product than someone buying a Mac Mini (the price point helps to assure that).
I know a lot of you want it so you can upgrade graphics cards, but Apple isn't going to be writing the drivers for those anyway, so what does it matter?