The installation of the new Java for Mac OS X 10.5 Update 4 failed on my old PowerBook. My first thoughts were, that it had something to do with the old PPC architecture. But the release notes still mention PPC compatibility. Repairing disk permissions didn’t solve the problem either.
One possible solution to fix the installation problems reported on Twitter is to download the update again manually and install it then. You find the download at Apple’s support pages:
http://support.apple.com/downloads/Java_for_Mac_OS_X_10_5_Update_4
This actually solved the problem for me. And what can I say? The update actually feels better than before! On both of my machines, image rendering seems to be smoother. If you like, check out my photo gallery, which is (currently) based on a Java Applet.