I don't think that's a huge concern to Apple when it comes to a machine like the mini. They axed it on their most popular system, the MacBook after all.

I'd love to see a display-less machine between the mini and the full towers

I just don't know how they'd position it within their existing lineup, especially in terms of price. One way would be to offer something more than the equivalent of an iMac without a display. Maybe quad-core and in three configurations. Up to 8GB of memory should be good.

Dual-display graphics built-in (card in slot), two PCIe slots, one of them being a 16x slot holding the graphics card, two hard drive bays, 2 USB up front, 2 USB in back, FW800 and FW400 up front and again in back, digital and analog audio in back with headphone and speaker up front. GigE in back along with an ExpressCard slot in back as well (for WiFi, etc..)

All in a case about 1/4 the size of a Mac Pro. I think that's a pretty modest description.