Yuki Sonoda has just announced the release of Ruby 1.9.1 Release Candidate 1 on the ruby-talk mailing list:
This is a release candidate of Ruby 1.9.1, which will be the first stable version of Ruby 1.9 series. Try it early and have a experience of modern, faster, with clearer syntax, multilingualized and much improved Ruby world.
We have fixed 72 bugs and implemented some features. If you encounter a bug or a problem, please let us know it via the official issue tracking system.
If you're a Ruby developer, particularly if you have any libraries out in the field, getting up to speed with Ruby 1.9 is becoming a necessity. There are still bugs to be squashed (recently, for example, I found one in edge Rails on Ruby 1.9.1 beta - now fixed). Some libraries do not compile with Ruby 1.9 at all yet. Yet Ruby 1.9.1's performance and general stability are so encouraging that popular migration to Ruby 1.9 is certainly on the horizon.
When Ruby 1.9.1 finally drops and the Ruby 1.9 branch is considered production ready, we'll be doing a big round up of Ruby 1.9 news, links, and resources, but till then you might find Dr Nic's guide "Future proofing your Ruby code - Ruby 1.9.1 is coming" to be very useful. Sam Ruby's OSCON presentation on Ruby 1.9 should also prove illuminating if your expose to 1.9 has been minimal so far.
(Update: Ryan Grove has written How to compile and install Ruby 1.9.1 on Mac OS X Leopard - a handy guide to downloading and compiling Ruby 1.9.1 for yourself on OS X.)