I finally got time to install Windows XP last night on my MacBook via Boot Camp. All drivers used were Apple's Boot Camp drivers. Here's an initial report regarding the battery life and power consumption.
I downloaded and installed MobileMeter to monitor power consumption and CPU frequency. Was having trouble to get a correct frequency reading, but after changing a setting, it was 1.6 GHz. This was on default laptop power manangement mode in XP. It seems this CPU cannot go lower than 1.6 GHz (multiplier 6x), which is disappointing. As a sidenote, I have been unable to get any frequency reading in OS X. If anyone knows a program that monitors actual frequency, and not load, please inform.
The actual power consumption of the notebook were hovering around 15-16 Watts, when basically doing nothing, and having the screen at the lowest brightness setting. That's is very high. I disabled BlueTooth, Ethernet and iSight, and it came down to 13.0-14.0 Watts.
I turned the Nvidia Powermizer on and off to see if it actually mattered, and indeed, when disabled the wattage increased to 15.6 Watts.
But other than that, I don't see what other obvious out of the box things one can do to lower the power consumption. I totally lack the tools and knowledge to find out what hardware is clearly drawing more power than it should (and compare to what it draws in OS X). My battery life was showing around 2.30-2.45 hours when doing nothing and quickly decreased.
I can tell that from my experience the MacBook power consumption in Windows XP is about twice what it should be. This also confirms fine with that I get easily 5.30-6.0 hours in OS X, when basically doing nothing.
It is very possible to get the power consumption down to around 7-8 Watts on a machine like this. Several ThinkPad models with similiar specs reach these kind Wattage numbers.
I can only conclude that the MacBook hardware, it's EFI, Bios emulation, and Apple drivers, do not play well with Windows. Again, this is really not a problem with Windows. And I doubt Apple ever will put effort into power management under Windows.