A new beta is live on the "experimental" branch on Steam. This is mainly meant for the XML planet builders out there to do some testing for the upcoming 0.9.300 release. For most of you, I recommend avoiding it for the time being. If you do use it, I'd recommend backing up your UserData folder prior to launching the game on Steam. Your game states will not automatically downgrade if you try to go back to a previous version.
Release notes and further details can be found here