Python and Ruby are not so different that I really see the purpose of this. Compiling Python and/or Ruby to the Java or .NET runtime has clear benefits, but what is the benefit of this?
The main benefit, aside from that it's sort of cool, is that you would be able to write for Google App Engine in Ruby. Personally, I think that the similarities between Python and Ruby (particularly their bytecode similarities) makes Ruby the most obvious language to be added to GAE next; but there isn't anything quite as easy as GAE for getting your Ruby app off the ground right now.