The rcov_plugin project is a rails plugin for rcov that adds some useful rake tasks to your application. And since I'm currently working in a JRuby project it made sense to use this plugin.
The thing is, among other stuff, an rcov report from a JRuby project includes some files that shouldn't be there at all, plus you also need to change the way you call rcov as such. Thus, I thought I'd contribute these changes to the plugin and my pull request was approved this morning - just install the latest version and you should be good to go.
It was useful for us here, hope it might be useful for you too.