My Friend John (BeeJay) has just managed to get Ubuntu running on his oldworld G3 Tower Mac. He's a Unix man but doesn't know that much about Macs which were made after the mid 80s. (He's got some of the early Macintosh and Apple computers in his collection). Anyway - after digging around a bit he's finally managed to get Ubuntu running.
Instructions for it are here.
I know several other people have done it but to my knowledge no-one yet has put a "how to" up. Since Ubuntu is designed for a modern "new world" Rom it was a bit of a challenge but he seems to have done pretty darned well. The only drawback compared to a later machine is that you have to keep a Mac boot partition and a Mac OS installed so that BootX can do it's thing.