Microsoft Windows Server 2008 R2 can report both a server's power consumption and any power cap under which the hardware is operating.
Not all servers have the hooks needed for reporting system power consumption and power caps, so Microsoft added a certification qualifier in the Windows Catalog called "Enhanced Power Management". This extra flag identifies servers that surface a power meter and reveal any power cap. HP ProLiant blades are one of the few server families listed; IBM and Dell servers are not.
On a ProLiant, an electrical (aka dynamic) power cap will be enforced by hardware regardless of any processor power control techniques being used by the operating system or server firmware. That means that you can still use things like Power Regulator for ProLiant or Windows Processor Power Management to adjust processor frequency on-the-fly, and the power cap will still be ensured.
Here's a screenshot of Perfmon showing a power cap in action on Server 2008 R2. I used a lightly configured BL490c G7 server blade. I set a power cap ("power budget" in Microsoft terminology) at 75 watts, then launched a processor-intense benchmark.
As you can see, perfmon shows the system power (blue line) rising up to the cap (the dotted red line), and then power consumption flattens out as the cap kicks in. I manually released the power cap via iLO at the point where the red dotted line drops off. The server quickly began consuming more power (as the processor frequency, the green line, rose).
Another snazzy power-related feature of Server 2008 R2 is called Processor Clocking Control. I'll follow up with a blog about that technique, which was jointly developed by HP and Microsoft engineers.