JRuby
Compatibility is *hard*. I'm not talking a little hard, I'm talking monumentally hard. Ruby is a very flexible, complicated language to implement, and it ships with a number of very flexible, complicated core class implementations. Very little exists in the way of specifications and test kits, so what we've done with JRuby we've done by stitching together every suite we could find. And after all this time, we still have known bugs and weekly reports of minor incompatibilities. I don't think an alternative implementation can ever truly become "compatible" as much as "more compatible". We're certainly the most compatible alternative impl, and even now we've got our hands full fixing bugs. Then there's Ruby 1.9 support, coming up probably in JRuby 1.2ish. Another adventure.
IronRuby
But there's some peril here too. IronRuby is largely still being developed in a vacuum. Perhaps in order to have secrets to announce at "the next big conference" or perhaps because Microsoft's own policies require it, IronRuby's development process proceeds largely from all-internal commits, all-internal discussions, and all-internal emails that periodically result in a blob of code tossed over the fence to external contributors. The OSS story has improved, since those of us on the outside can actually get access to the code, but the necessary two-way street still isn't there. That's going to slow progress, and eventually could make it impossible for IronRuby to keep up as resources are moved to other projects at Microsoft. JRuby has managed to sustain for as long as it has with only two fulltime developers entirely because of our community and openness, and indeed JRuby would never have been possible without a fully OSS process.
Say I wonder if anyone has written a Ruby implementation in Python, hehe
Have you started working on your Ruby implementation yet? All the cool kids are doing it. It's remarkable how many implementations of Ruby are in the works right now. It remains to be seen whether the ecosystem can support such diversity in the long term, but at the very least they're introducing splendid variation. And there's a lot more to do with Ruby in terms of performance, scaling, and "getting things done". Ruby's future is looking bright, in no small part due to the many implementations. How's your favorite language looking?
