I was a little surprised to discover, after upgrading to Ruby 1.9, that dev-ruby/minitest does not support ruby19 target, since minitest is advertised as the default test suite for 1.9. I'm sure the maintainers have their reasons, though they are not especially clear from the ChangeLog. In any case, I hope this bug report shows interest in having ruby19 support, and that we could hear about future plans in respect to this issue.
Versions 2.0.2-r1 and 2.1.0 have support for ruby19.