I agree that this is a GAPING hole in the product lineup, but as you said, it's been there ever since the G5 and growing steadily. Maybe this has been Apple's intent and now the time is right the introduce the long-awaited "Mac Prosumer" (or as I like to call it, "Mac Plus"

).
Oh and by the way, I've "heard from a friend" that installing Mac OS X on PC hardware is a lot more work that it is worth and that drivers are near impossible to find. You have to be very meticulous about what hardware you buy, especially concerning WiFi and Audio. No doubt the knowledge base has grown since "my friend" tried it, but it's still a hassle. Plus it violates the EULA for OS X.
Builder Beware!
-Clive