If Rust 2.0 ever happens, I personally will advocate very strongly that we do a final release of the 1.x series that has all of the 2.0 features, but issues deprecations for the things that will be removed. In this way, if you're on 1.last, and you have no warnings, 2.0 is a seamless upgrade.
I don't want a Ruby 1.9/Python 3/Perl 6 situation.