I don't know about "not considered stable." The official MacRuby site claims the current version is "Stable for Cocoa Development." It's not on the iPhone, granted, but that's because it's banned, so nobody wants to waste time on it.
The MacRuby team reckons it's "stable", in the sense that the API shouldn't change too much in the future and it probably won't crash. But there are good reasons Steve might not yet consider it mature enough for the iPhone. For one thing, gems don't work yet.
Not so. The MacRuby team had already thought up a number of ideas for working around the lack of a garbage collected runtime (the most promising was similar to the direction Apple's going with Objective-C, where retain and release happen automatically based on policies). They never got implemented because Apple instituted the ban on programming languages right around the same time.
