Apple has finaly enabled Java 1.6 on the newest Mac OS X Leopard release by offering the “Java for Mac OS X 10.5 Update 1 for 64-bit Intel Macs” update.
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.
You can find more information at Apple’s official site.