Getting that message running rake test (or individual unit tests) from within RubyMine. Works fine from command line, and was working within RubyMine using jruby 1.3.1. Started happening with jruby 1.4.0 sdk. I'm using language level 1.8, so I don't think I need the test-unit gem.
Was using beta 3. Just upgraded to the 2.0 release; didn't help. I'm running Windows XP. My teammate running on a Mac doesn't have this problem.