MOST osgEarth 2.x earth files will continue to work as-is, so hopefully the transition for earth-file-only users will be relatively smooth. (That doesn't mean you shouldn't upgrade your earth files -- but this should help with the move.)
Advanced users with custom TileSource or Layer implementations MAY need to do some extra work to bring their code up to work with osgEarth 3.0. We will provide help as necessary in the forum or on Github.
The GitHub repo transition will proceed as follows:
* The 2.x code base will live on in a branch called "2.10" -- this branch already exists. If you plan to stay with osgEarth 2.x, you should switch to this branch NOW!
* We will merge the 3.0 branch into the master.
* We will create one or more Release Candidates for osgEarth 3.0, culminating in a 3.0 release.
All this (except probably the final 3.0 release) will happen in the next couple weeks. Thank you to everyone for your continued support!