Scala does not follow the usual version number conventions. Scala 2 was a total re-write from Scala 1 (the compiler was ported from Java to Scala). Scala 2.8 is a major, backwards-incompatible release with Scala 2.7. Scala 2.8.x releases will be at least backwards source-compatible. (Binary compatibility is more challenging in Scala, because of how traits work, but the Scala team is allegedly tackling this problem for 2.8.1. We'll see.)

They decided to make some backwards incompatible changes after they had already announced 2.8 as the next release. They would have made it 3.0 but everybody was already calling the next release 2.8.