Will it be easier to implement and manage multithreaded apps with MacRuby? MacRuby's threads are exactly the same as Ruby 1.9. Nothing less nothing more. When you say "easier", if you mean easier than ObjC perhaps a little bit, but that's all.
And will MacRuby be eventually rolled into iphone Dev? Or is this one of those sensitive Apple NDA iPhone issues? I think Laurent would not be able to reply about this one, but it looks like you should not wait too much for an iPhone version of MacRuby. First, the iPhone is much slower than a Mac so MacRuby would be a little slow. An other problem is that autozone (the GC used in ObjC 2 and MacRuby) does not seem to have an ARM version. Of course it's probably doable (and maybe someone in Apple is working on it, I have no idea) but well for the time being I would not count on it.