This Java for Mac OS X 10.5 Update 1 adds Java SE 6 version 1.6.0_05 to your Mac. This update does not replace the existing installation of J2SE 5.0 or change the default version of Java.
For more details on this update, please visit this website: http://docs.info.apple.com/article.html?artnum=307403
Please note what the aforementioned article says:
This release is only for Mac OS X v10.5.2 and later, and should not be installed on earlier versions of Mac OS X. This release is for 64-bit Intel-based Macs only and cannot run on PowerPC-based or 32-bit Intel-based Macs.
If you don’t want to wait for Software Update to refresh for you, just go here:
If you have a developer preview of Java 6 on your system you’ll need to remove the
/System/Library/Frameworks/JavaVM.framework/Versions/1.6.0 folder and the
1.6 alias (or ‘symlink’) if it exists.