The first bits of IronRuby are available. I’m obviously going to have a look at this.
One very good news is that Microsoft is going to accept source code contributions to the IronRuby libraries and host the project on RubyForge. They intend to fully open the whole project once the DLR will have reached version 1.0 and will be stable enough.
Update (2025): IronRuby was eventually discontinued. But my early interest in bridging Ruby and .Net led to years of productive work mixing both ecosystems — from testing .Net code with IronRuby to prototyping VST audio plugins with JRuby.