You probably can't save much power on Model B by underclocking. Underclocking is possible, it's really very similar to overclocking, but in reverse (Hint: Overvolt settings also accept negative numbers)
You can't save power by underclocking the CPU because the lan chip uses most of the power, so it's really a lost cause.
To save power on a battery powered application, you really need to remove the LAN9512 chip, which amazingly, there is a page describing here:
http://freneticrapport.blogspot.com.au/ ... re_24.html
It's a tricky bit of soldering, requires some special tools and can probably easily overcook your Pi. You also lose one USB port and the ethernet. (I assume that a battery powered application does not need ethernet)
According to some reports, you can reduce the power consumption of an idle Pi by about 3x without the lan chip.