About a year ago, I "borrowed" the battery from Christina's MacBook to extend my "life", while I am on the plane. The Mac (being the Mac) has a nice feature, which will hibernate the machine when the battery gets low. This allows you to change the battery without rebooting.

I was looking for a way to hibernate the machine on demand. This is also useful if you want to conserve battery power (e.g. while one a plane). The sleep mode is a drain on the battery, because the RAM needs to be kept alive/refreshed. This is especially a problem, if you have a lot of RAM.

Found the following applescript. Seems useful.