Well crap! Sometimes things don’t work out how you planned. I was hoping to have Rcov ported to JRuby by the end of the week, but it looks like that’s not going to happen. The main problem is that the SCRIPT_LINES__ constant isn’t implemented in JRuby. Since RCov depends on that, it kind of throws a wrench in the works (here’s the JIRA issue). So, on to plan B: creating a SCRIPT_LINES__ patch for JRuby.
UPDATE: I think the JRuby guys are working on this whole SCRIPT_LINES__ thing. Those guys rock.