The detail that Intel hasn't addressed relates to operational ownership of this microcontroller, and that's a particularly sticky point for me. As processors become more malleable, who gets to shape them, and who gets to shut the door to further changes? BIOS? Boot loader? Kernel? Device driver? I've addressed the opaque, proprietary control that independent BIOS vendors, system OEMs, and Microsoft exert over processor and device registers that have a dramatic impact on performance. Nehalem's power controller has similar reach with regard to power utilization and scheduling.
To be blunt, it's a resource that Microsoft will want to own, or reserve the right to disown by overriding the power controller's settings with Windows' more primitive run-time controls. This is already seen in AMD Barcelona servers running Windows Server 2008. Left to itself, Barcelona can manage bus and core power beautifully without Windows lifting a finger. That's core to the CPU's design. Yet Windows can ignore BIOS and user-defined power settings, and there's no checkbox to disable Windows' power state manipulation.
There should be. By putting a microcontroller in charge, Intel's gotten the best kind of religion with regard to power control. Nehalem reads like a CPU with a built-in greenness dial. But you'll never feel it if BIOS and the OS lock it down, and if Intel doesn't provide developers and users with the means to grab control at run-time. If I want to run my server on one core over the weekend, I should be able to do that.
One thing that a microcontroller could be trained to do is hoodwink the OS into believing that it has control over the system's power state while the power controller does what it, or a savvy system owner, knows is best. Nehalem's power management controller is Intel's engineering secret weapon and a welcome advance in x86 technology. Let's just hope that Intel keeps it open so that it doesn't fall into the wrong hands.