Keep in mind that IronMonkey intends to operate based on bytecode conversion/manipulation. Using the CLR provides a single bytecode set to convert from. Ruby currently doesn't have any sort of bytecoded VM (though Rubinius and YARV will both be that), and even if it did its bytecode and Python's would represent two separate targets. This, I daresay, is the end problem.

So the question really becomes `what about Java bytecodes?' More specifically, there is both an JRuby and a Jython. Then again, I don't know that JRuby compiles to bytecode the way that IronRuby intends to. The same goes for Jython.