Linux on a pre-Intel Mac meant using Yellow Dog Linux, the most stable and most popular distribution (imho) for the PowerPC architecture. I remember dual-booting one of our Emacs with Mac OS X and Yellow Dog Linux. :)
With the Macs going Intel (ergo, having more Linux distribution choices), Yellow Dog Linux (YDL) still continues to develop for its Power architecture.
Today, YDL powers the Sony Playstation 3. Yes, that Sony game machine that competes against Nintendo's and Microsoft's game consoles. Linux Insider takes a detailed look at what YDL is up to today. Check it out and learn why MIT students are playing with their PS3's in the lab. :)