"[...]but [the Mac Pro] has to be viewed with an eye on the much lower cost of a similarly specced Dell PC for example."

Could you cite the specific "similarly specced" Dell machine that's more affordable than the Mac Pro? I'm in the market for a couple of workstations, and unless something dramatic has happened since I did my initial round of price comparisons a week or two ago, Dell's workstations were dramatically more expensive than comparable Mac Pros.