According to http://guides.rails.info/getting_started.html Rails 3
requires either Ruby 1.8.7 < p248 or Ruby 1.9.2dev because everything
in between has bugs. That's quite unfortunate seeing that the MRI core
developers haven't released any fixes for the bugs yet, yet Linux
distributions are eager to package the latest version (p249).
We've backported the bug fixes that affect Rails 3 to p249 and
included these in the latest REE release, 2010.02. I think the
documentation should be updated with this fact.