I do want to support Leopard, but none of my PPC machines run it and I don't want to set one up specially for that. I do have a solitary Intel Mac mini running Leopard that I use for development purposes, however, and I might test it under Rosetta (if it runs there, I would be surprised if it didn't run on a real PPC). Now that 10.7 is obviously in play, the end of Safari for 10.5 and therefore PowerPC is also clearly visible.

The PhyrePhauCs one took me a while to get interesting idea though. Any others?

The compatibility portion I am most worried about is plugins, specifically Flash. That said, 10.1 looks like the last PPC Flash anyway, so it may all be a moot point soon.