It's not quite as dire as all that. "Unsupported" in this case just means it's not officially sponsored by Canonical, the main financial backers of Ubuntu. It's just a community driven recompile of the current source for PPC. It's fairly enthusiatically community supported, like other *nix distros.
My comments about video cards shouldn't discourage you. If the card is an OEM install on *any* supported Mac, then it should be recognised by the installer magic. As for the G4 upgrade, I would hazard a 95% guess that you'll have no problems there either, seeing as they are such a popular upgrade amongst the performance nuts who do this kind of thing *ahem*. A quick google should confirm that.
As the range of add-on hardware for Macs is so much smaller than for PCs, drivers should be no problem - they're pretty much all there on the disk, integrated from one of the many PPC unixen. My guess is a 9600/G4 with a zippier video card would actually be one of the *most* common configurations for PPC Linux. Unless you're sporting some really weird, obscure hardware, I would say - suck it and see!
My experience with it has pretty much been - you stick it in the CD drive, click install, follow the prompts (if any), and you're in business. Besides, you can always run it off the CD to check it out, then install later.