Apple has been criticized for not including Java 1.6 into Leopard. The current default is Java 1.5 when all the other platforms (Windows, Linux, Solaris etc) have access to Java 1.6...
Since SUN opened the code of Java it is freely available for porting and modifications. What should happen eventually happened: somebody took the source code and ported it to Mac.
The result is "SoyLatte: Java 6 Port for Mac OS X 10.4 and 10.5 (Intel)" by Landon Fuller. The performance seems to be pretty good too. See the benchmark comparing Ruby on OSX and JRuby on Java 1.6/OSX by Charles Nutter.
With this release, OSX users are getting closer and closer to a stable release of Java 1.6 for their machine (only Tiger or Leopard).
There is a developer preview from Apple, but it only works under Leopard and you need to be registered as a developer to access it (registration is free).