Interesting results, thanks for sharing. :)
I would point out though that the biggest gains using Powerd are when your CPU has some sort of dynamic cpu frequency/voltage control that has a freebsd driver, Enhanced Intel Speedstep or AMDs powernow for example. The D2500 does not have speedstep. The Pentium-M does and it showed an overall reduction of system power of 5W (15%) in my testing at idle. I agree though that the lowest power state is in the higher 'C states' and if you have that enabled in the bios, and your ACPI table is complete and correct then you are probably already running at least power. Speedstep 'P states' become important if your CPU is running at a moderate load continuously when there is not much time to use C states. http://www.overclock.net/t/1058894/intel-acpi-guide-c-g-s-p-states-and-ocs
It worth noting that the Pentium-M is a special case here because the frequency/voltage values are hard coded into the est(4) driver and hence cpufreq can use them even if the ACPI table does not play nicely with FreeBSD (many, many bioses!).
I have a box in which I replaced the CPU, a P4 2.8GHz, to save power. I didn't need that much processing power. You can save quite a bit by simply using a lower speed P4. I then used a P4-M, which is pin compatible, and saved some more. The P4-M has speedstep though it is very crude, only two steps, but my own board doesn't support it. You are using a laptop though so I would investigate that.
Steve