For new world Macs (anything after a Beige G3) Yaboot is the boot-loader to go for, it is possible to boot OSX and multiple (if your feeling sadistic) installations of Linux using it. I don't know how well yellowdog linux guides you through set-up these days, last linux I installed was Gentoo on a PPC box and getting everything working was a challenge. Though mainly due to some rather bad choices when configuring my kernel. However Yellow Dog is generally meant to be easier and has a nice GUI for walking through the setup. Be aware that linux on PPC is generally a bit more of a challenge than on BIOS based intel machines.
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Burt
I break stuff.