Not a lot of detail that I have found outside of this twitter, but it looks like the naysayers that said a) Microsoft would never support Rails, and/or b) IronRuby will not be able to run Rails, may need to get a big plate of crow. Still hunting for more concrete info, but if true, this is a great day. I am not a Ruby guy, nor a RoR guy, but I love the fact that Microsoft and the .NET Platform is showing some flexibility in supporting different technologies and more importantly, supporting developers that typically have about zero interest in the Microsoft platform.
To give credit where credit is due, I scammed this bit of news from Javier.
Update: You can get more on this announcement from John's website - http://www.iunknown.com/2008/05/ironruby-and-rails.html