Rails 2.2 is now compatible with both Ruby 1.8, Ruby 1.9 and JRuby out of the box. As more interpreters and versions pop up, the is an important thing to focus on. As all of the interpreters should keep on the same page, so too should their most popular app.
Note that while this applies to all the libraries officially part of the Rails distribution (like ActiveRecord, etc), it does not apply to other libraries commonly found on Rails applications. This includes things like RedCloth, all of your favorite acts_as_* plugins and any other gem or library. Compatibility with the various interpreters is completely up to their respective authors.