JRuby 1.1.2 is going to be released (probably) today and it fixes a bug with $~ constants (Regexp.last_match), which made autotest finally work!

jruby -S gem install ZenTest
cd myproject
jruby -S autotest

I’ve previously submitted a patch to ZenTest guys. It makes the use of autotest possible with any JRuby 1.x, but now, with JRuby 1.1.2, autotest should just work out of the box!