Recently latest Java has been partially ported to MacOSX. Still no sound support, UI libraries still require X11, no mach thread priority and other odds. However, if you run JRuby or JPython on top of it, it works faster than originally written implementations on C++.
I did not tested JRuby, but latest JPython 2.2.1 runs Fibonacci test 10% faster than CPython 2.5.1 on the same machine.
Links:1. Java 6 port for MacOSX: http://landonf.bikemonkey.org/code/macosx/
2. Jython: http://jython.org/
3. JRuby: http://jruby.codehaus.org/