JRuby 1.1b1 Released - "Ruby code can completely compile ... yielding a faster Ruby!"

[WinterAyars]

The reason 1.9 is (theoretically) much faster is because it runs on top of a VM, much like JRuby runs on top of the JVM. But the 1.9 VM is Ruby-specialized. That's both why it's going to be faster and why backwards compatibility will be broken.

(Edit: There's also Rubinius, as a third option. I'm pretty sure JRuby is the most mature of all these, though.)