I've been thinking about and complaining about headless macs for quite a while now, and I've come to the following conclusions:
The premium "mac-tax" i'm willing to pay is about 10-15%. A 20" ACD was $700 when I bought my 20" dell for $395, easy decision. But what I want is a "normal" computer. The equivalent of a mid-high end peecee, single dual/quad processor, 4 memory slots, room for 2-6 HDDs, a few PCIe slots, and the regular connections. Such a Dell would cost around $1500 with high end Conroe, 7600 video, 2 gigs of ram and a 500G HDD. Therefore, tack on a 15% mac tax for OS X and a pretty case, and I'll pay $1725 for it. More than that and I'll use uglier alternatives that do the same job.
It'll suit my needs well, be 85% as fast (or closer, depending on the clock speed) as the mac pro for nearly everything, and will last me nearly as long because I can put in 8 gigs of ram, 2 or more HDDs, a new video card when the original gets dated etc. It will also retire in 7-8 years to make a nice server or similar, which no iMac etc will ever make.
The problem isn't that the 'mac tax' is too high, it's that Apple is correctly pricing the wrong products for most users. The mini isn't expensive for what it is (a very specialized laptop motherboard and processor crammed into a fancy little case with laptop HDD etc), it's expensive for what it does. Apple could make a larger mini with off-the-shelf desktop parts that would outperform the mini for somewhat less, attracting tons of regular computer users who would prefer OS X if not for the high price of the hardware. The mac pro is an even better deal, almost no "mac tax" at all, but unfortunately it's a full blown workstation that's overkill even for some full-time professionals. Apple should make such a high-end option, but a version 90% as capable at 2/3 the cost would surely attract more home users as well as more professionals.
But reality being what it is, I do not see a new desktop model, not even a cheaper, desktop component based mini. What might be possible is a less expensive Mac Pro. A small drop in quad prices when octos are released would be great, but even better would be a 2-tiered mac pro, resembling the structure of G4 and early G5 powermacs. This would necessitate a new motherboard though, which I don't think Apple would do without giving it a new name and enclosure.
Final thought: Perhaps the Mac Pro was just a temporary solution for Apple. They knew CS3 would take a while, and that intel would offer better alternatives to double-price lava-hot ram in the future, so they reused the case and made only 1 tier with some basic processor/graphics alternatives. When CS3 is universal and the creative world stirs from its transition-induced slumber, they'll release a more broad line of Mac Pros, making darn sure they capture every graphics shop, ad agency, photographer, video editor, print house, audio editor and independent artist out there. New case? maybe, but it won't (shouldn't) be smaller or lighter. Options? 2-3 tiers, dual and quad core conroes and xeons, dual processors or single configurations for xeons, regular or buffered ram, more graphics card options, all in the same large quiet attractive case with 4 drive sleds, 2 optical bays etc.