That's true, but I don't buy that argument: look at how fast everyone has updated their gems to be Ruby 2.0 compatible. The Ruby world likes running on the edge; if MRI forced us to consider thread safety, we'd learn about the issue much quicker as a community. Since we'd be forced to. ;)

Oh I don't doubt that the community would move quite quickly to support it, but it certainly wouldn't be without some bumps along the way!