Java SE 6 was finalized in December 2006, and offers many, many enhancements to Java SE 5, in the areas of compatibility and stability, ease of software development, the appearance of desktop applications, and compatibility with XML and Web services. There were also tremendous gains in performance: Applications just ran faster on a Java SE 6 implementation.
However, when Mac OS X 10.5 came out, a lot of people (myself included) were surprise that Apple included Java SE 5 with the operation system. That meant that Mac developers, and Mac users, who wanted to write Java SE 6 applications or run them, were just out of luck.
That has finally changed, as Apple released a Java SE 6 runtime as a free enhancement to anyone running Mac OS X 10.5.2 on a 64-bit Intel-based system.
Translation: If you're using Mac OS X 10.4.x, or an earlier version of Leopard, you're out of luck. Ditto if you're using a PowerPC-based Mac, or a 32-bit-only machine, such as one based on the Intel Core Solo processor or some of the early Core Duo chips.
I assume, but don't know for certainly, that Java SE 6 will be included in the Mac OS X 10.5.3 upgrade, whenever it comes.
So, Apple -- thanks for not making us wait for Mac OS X 10.5.3 to get the Java SE 6 upgrade! But what took you so long?