You can get Java 5 for Mac OS 10.4 right now if you're a developer.
It appears that you can also just copy the Java-related files across to Panther and it works fine there too, even though you are prevented from installing it directly.
A google should give you the info you need.
Also, Java programs are generally cross platform so you could just test it on linux.
Comments
Sun's implementations won't work, they're only for Linux and Solaris.
Presumably Apple's working on JDK 5.0. Rumor has it that it'll be included with 10.4 and may require 10.4. But those are just rumors.
Roll on 10.4
Dobby.
It appears that you can also just copy the Java-related files across to Panther and it works fine there too, even though you are prevented from installing it directly.
A google should give you the info you need.
Also, Java programs are generally cross platform so you could just test it on linux.