Beyond the obvious benefits to developers, what interests me is that Microsoft is now bundling an open source component into their commercial offerings.
This is a first time for Microsoft.
With IronPython they continued development of an open source project in house. With IronRuby, they were open to external contributions to the libraries of IronRuby. In both cases they were not taking external code or contributions directly into their core products.
Hopefully they will start using more open source code in their products. Maybe one day they will bundle Mono's Cecil or Mono's embeddable C# compiler.