Download the J2SE 5.0 for Tiger file from the Apple website. Drag the resulting .pkg file onto the Desktop, control-click on it, and choose Show Package Contents from the pop-up menu. Navigate into Contents and trash the InstallationCheck file. Finally, run the pkg file, and there ya go!
Now you can put the new Java onto 10.3, even though Apple says you can only put it on 10.4.
[robg adds: I have not tested this one, and I have no idea if there are any dependencies in 10.4 that would prevent the new Java from working properly (even though it's installed) under 10.3. If you know more, please feel free to comment...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051117173104762