What does that mean? It means that if there are any bugs in the Ubuntu code that really break things on the PowerPC side, the release will not be held up to fix them. Furthermore, the Ubuntu PowerPC Architecture Team will be responsible for supporting the codebase for the PowerPC distribution. If you would like more details about this decision check out the PowerPCReview page on the Ubuntu wiki.
I have fooled around with Ubuntu, and it is a nice, solid OS (heck, Peter Rojas loves it and that's gotta count for something) but I can't say this is a surprise. Apple isn't pumping out any PowerPC machines anymore, so clearly support for that chipset will wane over the years.