I've been developing Scala full time since 2.7. I have not had a release yet that didn't involve changing code to recompile to each new version.

I also have Java projects that were written in pre 1.0 releases that have updated to 1.7 without a single code change.

This is due to a different set of priorities. Each of those priorities has different costs and benefits but to act like Scala's lack of backwards compatibility doesn't have a cost is unfair.