Some Linux distributions are okay for PPC arch. And some are not. Most installations are either x86 or x64 based, meaning that they have been pre-configured to install on those architectures. Fortunately for you, there are plenty of PPC-based Linux distros. Yellow Dog is a good example, for more go here:
http://www.themacforum.net/forum/ind...?showtopic=425
The best thing to do is to partition your hard drive, so that you can run OSX and Linux together. That way, if something goes wrong, you ain't SOL.
By the way, why would you want to install Linux on an iBook? Just about any Linux program can be built for OSX. And there's a system called Fink that lets you install binary packages. You just need some tools to get started, like the developer disk and X11, which can be downloaded for free.