go for it...
funny timing.. i'm actually in the middle of setting up debian on a powerbook g3 (lombard 400mhz). so far it's looking good - x is up and running, modem is all set up, actually it was a really pleasant installation experience, especially considering this machine died a slow death running os X! even with the standard non-configured kernel i've put in so far it's really speedy - blows OS X out of the water of course, but then everyone knows OS X is meant for power, not for a lightweight operating system. anyhow, here's some pros and cons of linux on macs IMHO:
pros:
1) apple hardware is nice... once it's up and running it's an undeniably sweet little system, especially on apple laptops...
2) linux takes advantage of the niceties of apple hardware better than apple's own operating systems, or at least, it's light enough that you realize just how fast your processor is - you'll quickly see just how fast the G3 is once you switch to linux from OS 9..
cons:
1) linux is not as well established on macs as it is on x86, meaning info and support are harder to find. i set up linux on a toshiba tecra and there's probably 15 sites out there with good info on how to do it. for my pb g3 i've found about 3 sites, nowhere near as much information.
2) the one-button-mouse thing is kind of a pain in linux, which is pretty much built for two/three-button mice. you can emulate it on linux, sure, but it's not a pretty sight.
3) cost - you can run linux on comparable x86 hardware for about half the price, and linux really isn't picky about what you're running it on. (of course, you're also looking to run OS X, so obviously this won't make a difference to you).
hope this helps! feel free to e-mail me if i can be of further assistance. some cool linux apps to check out: pd (synthesis/sound-mangling software, basically a free version of max/msp!), gem (video extensions to pd).. and lots more!!!
best,
nick