I think Apple is making a mistake by not providing a mini tower. I have a 3+ year old 2ghz G5 at work. It will need replacing at the end of the year. I know for a fact that if I asked my boss to get a MacPro he would take one look at the price tag and scream. Thats even with a University purchase price (edu discount and minus VAT as well). And why would he react like this? Because we can get special price Core2 Duo Dells for about £600-800 (usually with a screen as well). I'm a web designer so a tower is an ideal rig. I need to have dual screens, ram/hard drive upgrade abilities. The iMac doesn't really cut it. I have a 24" iMac at home and while its nice to do design work on, having a 2nd display next to it is clunky. I have two 17" NEC's at work and they sit fine on the desk. And work would never order an iMac as they'd see it as just a toy computer.
Surely Apple can come up with a Dimension/Optiplex like mini tower that fits nicely between the iMac and the MacPro?
It is precisely for this reason why we won't see a mid-range tower. Why have it eat into Mac Pro sales?