After changing the link of the CurrentJDK to 1.5...
/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK -> 1.5
... I can use java 1.5 on 10.3:
$ java -version
HotSpot not at correct virtual address. Sharing disabled.
java version "1.5.0_02"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_02-56)
Java HotSpot(TM) Client VM (build 1.5.0_02-36, mixed mode)
So far, I haven't had any problems, but I'll investigate it further.

