That "completely new mobo architecture" would be a standard and very easily designed one. They could just reuse the mini or iMac mobo and make some minor tweaks, or even use one of the standard intel reference designs and do a little bit to make it more "mac". But it's a case of reinventing the wheel, the work has already been done. Cases are easy, especially a midtower, and support shouldn't be much different than any other mac they sell, since it would offer no features that aren't already available in other macs, just in different combinations.
I have to disagree here. Apple's current products don't use the components that would be put into a prosumer tower such as we're suggesting. They couldn't re-use anything from the Mini or the Pro. Although - as you suggest - a lot of reference designs are availible, it still means a completely new line of logic boards and supporting components. Apple has to source these components, integrate them and write EFI support for them - not to mention the hundreds of little tweaks they would have to do to differentiate from a reference design. Take the Mac Pro memory boards, for example. When I first saw them I wondered how Apple was able to implement them without signal degradation. It must have taken some smart cookies to figure all that out.
Many case designs exist, granted, but Apple has always tried to excel in this arena. They are constantly on the lookout for new manufacturing methods, and often regularly consult manufacturers to see what new processes they have in the works. I'm sure there's a large R&D section in Apple responsible for things like the now famous translucent acrylic casings, the two-toned (gloss and colour) plastic, the seemless aluminium shells - just about any feature that makes a Mac a unique piece of machinery. IF Apple were to re-use standard PC manufacturing techniques for their MPL we would be bitterly disappointed. If they choose to re-use a design theme they already have in production (let's say a half-height Mac Pro aluminium enclosure) they would still need to design and build the new tools that would produce these enclosures. The tools themselves aren't cheap, and considering they would need to still maintain their old tools (for the remaining product lines) this is more additional cost that the user doesn't see.
I challange any user on these forums to create a case similar in quality to a Mac Pro, iMac or even Mac Mini. I suggest you would need access to an industrial CAD-CAM machine, not to mention considerable skills.
I may come across as strongly anti-MPL, but on the contrary I would love for Apple to produce one more then anything. I have seen many good arguments against it, and I'm choosing to back the "against" side. Incidently, I find that the worst argument against is that it will "eat in to" other products. If Apple has a shred of business sense, it can quite easily tweak features and prices of these products so that there is no contention over a market - increasing the specs and price of the Mac Pro, for example.