The Microsoft team building Ruby for the .NET Framework has released the first source code for IronRuby. See John Lam on Software: A first look at IronRuby for the announcement. The IronRuby source code was released under the Microsoft Permissive License.
According to Lam, they are accepting contributions to the libraries, and planning to get IronRuby onto Rubyforge by the end of August. Why Rubyforge and not CodePlex? So that we can access it from Subversion instead of TFS.