We're excited about Ruby 1.9. It's fast and stable, and brings nice improvements for programmers. But there are obstacles to adoption---mostly gems, plugins, and tools that don't support 1.9 yet.
So we're working on the things Relevance maintains, to get them on board. We're doing it by dog-fooding: We've got one customer project that we're actively testing on both 1.8 and 1.9 (although deployment is still on 1.8 for the moment). That's helping us see where the holes are, so we can fix them. We're already working on Ruby 1.9 support for RunCodeRun, and investigating the best way to interface rcov with 1.9.
Now one more piece of the puzzle is in place: on our last open-source Friday, Chad upgraded Tarantula to support Ruby 1.9. If you're trying out Rails on 1.9 (or even if you aren't), get the latest version (0.2.1) of tarantula from github:
gem install --source http://gems.github.com relevance-tarantula
Then add the plugin to your app and add some fuzz-testing goodness to it!