Why would Apple do this? It makes no sense. It would instantly crash the value of Macs. OSX machines would engage in an immediate race to the bottom and soon enough they'd all be plastic and icky.
If Apple certified a few high-end workstations for Apple OSX, along with a limited number of high-value option cards (GPUs, Fibre Channel, 40GbE Ethernet, ...), the "value crash" would not occur.
The certified workstations would still be more expensive than most of Apple's own machines.