With the latest iteration of PowerBooks Apple just released, a new feature called Safe Sleep was introduced that Fabienne covered a couple weeks ago. In case you missed it, the short of the long of this handy new feature is that it works like the Hibernate feature of Windows: you can save a snapshot of what you're working on to your hard drive and completely power down the notebook - meaning you can swap out batteries and power back up to your desktop as you left it in about 10 seconds, or simply store the notebook without draining any power, like the regular Sleep feature does.
Since this new feature is so handy, some Mac users out there were determined to claim it for the rest of us and it appears they were successful: on Digg.com I found a blog entry detailing the fact that this new Safe Sleep feature is entirely software based - so they've figured out how to enable it for the rest of us and have posted a tutorial. (It's reported this works on iBook G4's and PowerBook G4's, but it isn't necessarily limited to those models.) As always: back up your stuff before doing this. It sounds like a fairly painless procedure (though I haven't tried it yet) but there is never a good reason not to back your stuff up. For those of you who try this out, please post your experience in our comments for the rest of the class.
Side note: I thought you could do something similar to this already with the 15" and 17" PowerBooks, where you could set the book to regular sleep, pop out the battery and you had 90 seconds of stored power to pop a new one back in. I've seen this demonstrated, but I can't find any documentation on how to do it. If anyone has info on what I'm talking about, I'm sure we'd all love it if you could post that too.