On the Ruby 2.0 announcement thread, Marcel filled us in on Ruby 1.9 compatibility work in Rails 2.0:

A lot of exploratory work and updates have already been made for 1.9 compatibility, mostly by Jeremy Kemper. So Rails 2.0 isn’t that far off from being 1.9 ready.

Hopefully this means that we will see a Ruby 1.9 compatible version of Rails a few weeks after its release.