I'm wondering if the hackintosh is the reason the MacPro is being left to wither on the vine...
I doubt it - not that this isn't a valid idea but simply because of conversations I have had with people (mostly creative professionals) who buy Mac Pros. A couple of reasons why:
1) When you're an established business, you don't go around breaking the EULA of the company your workflow is built around.
2) When your livelihood depends on your computer, you don't play with the possibility of important updates breaking your system.
3) Though Macs do fail, AppleCare is still a HUGE motivator for buying a real Mac Pro (this is the one that comes up the most).
Honestly spoken, the majority of people who build Mac Pro-type hackintoshes probably wouldn't shell out the $2500+ bucks it would take to buy a base Mac Pro. This is a general observation - obviously we have seen some KILLER builds on this forum, but generally speaking people who build hackintoshes do so to save money or to tinker. The Mac Pro is designed for the professional market, the majority of whom probably are still buying/considering buying a Mac Pro (although their sales will drop until they announce the new line later this year).
Just my casual observations, but they are backed with many conversations with Mac Pro owners in various professional fields.