I actually hadn't looked too closely at MacRuby. It is quite an achievement, but compared to (particularly) Lua it's just not fast enough. Compare that to LuaJIT and the differences are remarkable. If MacRuby could be made as fast as Lua then I'd be all for it, I just don't think it's possible given the language semantics.
I guess what I'm saying is that the idea of a dynamic language like any of Lua, Ruby, Javascript, etc. . . being the default systems language on iOS and Mac OS would be amazing; I just don't like Ruby's chances when paired against the other two.