The other opportunity is to just call some version 1.0 and then wait some amount of time (a year? two? five?) before making any breaking changes, which sounds less appealing to us. You can always continue to use an older version (and we'll backport serious fixes like we did in http://facebook.github.io/react/blog/2013/12/18/react-v0.5.2...) but we plan to continue developing React and hope that people will continue upgrading to new versions.
If you're concerned about whether it's production-ready (as opposed to future API changes), the answer is definitely yes.
I've been using 0.13 in production for half a year with no problems.