Java 6 on Mac PowerPC with Mac OS X 10.4
Note that in addition to not working on PPC, Apple's Java6 does not work on 32 bit Intel either. And there are other restrictions for the new Java6, leading to opinions such as this.
As of December 2009, a beta of OpenJDK 7 for OS X 10.5 PPC is available at http://landonf.bikemonkey.org/static/soylatte/ -- haven't tried it, and it's not quite what you were after, but it's the first I've seen so far of Java >=6 on any PPC OS X version! More details at http://landonf.bikemonkey.org/2009/12/index.html .
Someone yesterday recommended SoyLatte for PPC (Installing Java 6 on Mac OS), and in fact these instructions seem to support his theory (read the comments where someone claims it works on his PPC): http://blog.adsdevshop.com/2008/02/26/installing-the-jdk-16-on-mac-os-x/
YMMV :-) Or try building from source. Again, YMMV :-)