An update on the project status:
I've released a gem on the project page. It's named json_jruby for now (I've sent an email to Florian; let's see what he says), but once installed it goes just like the original libraries (i.e. require "json", not "json_jruby").
From my tests, after the usual warm-up, the parser's performance is amazingly similar to json-C. The generator is a bit slower, but I plan to investigate that.
By the way, I only targeted JRuby trunk. It works with 1.1rc1, but breaks on 1.1b1 (due to RubyHash#visitAll, it seems). If you think some particular version is worth targeting for, please mention it, I can take a look at that.
Nick and Marco, this version should be good enough for testing. Please report anything wrong, or if you need support for a specific JRuby release.