I just got Ruby on Rails 3.0 running under JRuby by following the explicit instructions here:

JRuby and Rails 3, Sitting in a Tree

I am pretty strong with Ruby on the command line, and have a preference for using JRuby where I can leverage technologies like JDBC, but as I do basically all of my web development using PHP I am not as strong on Rails as I should be, so am now on the lookout for a pet project to learn the framework somewhat.

Since I last looked at Rails around version 1.0, it will be interesting to see how it has changed with this third generation release. You can download Rails 3.0 from here:

RubyOnRails.org

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Updated 2021 : note that the above post was originally published in 2010, but is left here for archival purposes.