Well, you called my bluff. I have only theoretical numbers but in principle lowering the voltage can save a lot more power than underclocking because it's a quadratic vs. linear effect--and you don't give up any performance. The only limitation is the CPU/SoC of your N900, which means a (very) small risk of needing to flash and restore a backup if your FS gets corrupted while you test. It seems like the vast majority of N900s can handle Ideal.

My goal was to overclock while keeping within the power envelope of the stock settings. By using Ideal settings and changing the min and max to 250 and 810, i was able to do that. At all clock speeds except 810 MHz the CPU power consumption is less than with stock settings at 600. If i decrease the max to 600, it's only 63% stock at 600. At 500 MHz, Ideal settings have 66% the dynamic power of stock.

I used numbers from this: http://wiki.maemo.org/Overclocking

Note that i don't expect the method in this thread nor undervolting to make a significant difference in overall N900 battery life unless the user is regularly pegging the CPU at max (emulators, mostly): the radios, screen, etc. all consume power, too.