The Mac Pro has a TINY market: you're talking about a full tower Nehalem quad core with 3GB of RAM that starts at $2499. There is really only kind of customer for an item like that: someone running a computer based operation (audio studio, graphics designer or the like) for business. Any home user (and a decent percentage of the prosumer market) is going to be looking at the Minis which start at like $500. The only ones who even want Pros don't tend to care about prices; but what percentage of the market falls into that category (not caring about prices)? From a marketing perspective it's such a small niche they might be able to save money by scrapping the line.
Personally, I think they reached that point with the Mac Pros a couple of years ago already; now they keep the line going for essentially one reason: to satisfy the small niche of high end Mac users. Whether or not it continues to be cost effective to do that, even at the extreme markup of a Mac Pro, remains to be seen.