There’s an old imac under the table at bz’s house that’s gathering dust and I was going to try to put Linux on it. I’ve had a good experience with xubuntu running well on older hardware and having good, easy-to-use features (and it’s debian based), so I’m going to try that and document my experience here.
I had a little trouble finding the iso for PPC xubuntu, but eventually found it at http://cdimage.ubuntu.com/xubuntu/ports/releases/7.04/release/
This article gave some information on installing xubuntu on various platforms, and said that the firmware needed to be upgraded on the iMac before xubuntu could be installed. I found the firmware for this particular iMac (slot-loading, 350 MHz, 128MB SDRAM) at http://docs.info.apple.com/article.html?artnum=88010 (or maybe at http://docs.info.apple.com/article.html?artnum=75130) and instructions for installing the firmware at http://docs.info.apple.com/article.html?artnum=60385.
The machine ended up being bricked, but I thought I’d post this for future reference.