Late on Tuesday, Apple posted a Leopard only Java for Mac OS X Update 1. The update adds Java SE 1.6.0_05 to the system.
"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," according to Apple.
The reference page for the update notes that the release is only for Mac OS X 10.5.2 or later and is for 64-bit Macs. It cannot run on PowerPC-based Macs or 32-bit Intel Macs. [The Core 2 Duo is a 64-bit processor, the Core Duo is 32-bit.]
The update can be found in System Preferences -> Software Update or on the Apple Downloads page. It is 57 MB.
Note that Sun continues to use an idiosyncratic, dual nomenclature. Java 5 means Java 1.5, Java 6 means Java 1.6, etc.