Ruby 1.9.1 was never considered stable by many Rubyists. (Find '1.9.1' in this Yehuda Katz interview: http://www.drdobbs.com/blog/archives/2010/06/preparing_for_r... ... which was just the first reference I found.)

Mr. Katz advised in multiple interviews/tweets (if I recall correctly) to wait for 1.9.2, and the community tends to listen. Rails 3 was tested against 1.9.2, not 1.9.1; this provided yet another reason to wait.