It's true, there is a Gentoo port for PPC, which is mostly based on the fact that most of Gentoo is compiled on the very machine it's installed on. So it's mostly a matter of kernel and boot procedure.

As for the other distros, there has been a debian port for PPC for a long while, but I have no experience with it. There used to be SuSE for PPC, but they don't support it any more. As for Mandrake, they still carry on support for PPC, but only on alternate versions. That is, there is a 9.1 for PPC, there is not 9.2 for PPC, and hopefully there will be 10.0 for PPC.

You should notice that it's not the "main" distro which happens to run on the Macintosh. The major distros are compiled specifically for Intel hardware, and will not even begin to boot on a Macintosh. Macs have a very specific boot-up procedure which the distro has to take into account. You won't even get to the stage where the kernel panics on you. It simply won't boot.

So, what else do you have? We said YDL, Debian, Gentoo, Mandrake, and there is also a Slackware-based distro called Slackintosh, as far as I know.

Take a look at PenguinPPC for more info.