From what I'm Googling, it seems the only "official" way to get Java 1.6 on a 32-bit Mac is by upgrading to OS X 10.6. With 10.5, you're stuck with 1.5.

The Mac port of OpenJDK also seems to demand a 64-bit CPU. Are there any other open 1.6 JREs that might run on an ancient Mac?