One of the things that's obvious when you start building applications with Flash Builder 4 is that there's a new component architecture. One of the other things that's obvious is that the ActionScript language and the Flash foundation class library have both been changed for version 10 of the Player. For example the new version of ActionScript includes a new strongly-typed Vector datatype and the Flash foundation class library includes support for 3D transforms etc.. That's progress I guess. However applications written in the "new version" of AS3 supported by Player 10 won't work in Player 9. It's always like that with new releases of the Player and it's no different from in Java and .NET with one key difference. When Sun releases a new version of the JRE which supports a new language features or additions to the Java foundation class library they increment the version of Java. JRE5 runs Java 5, JRE6 runs Java 6 and so on. It's obvious that code written in Java 6 won't run in the JRE5. That's software release management 101, right?
It seems Adobe doesn't want to play by the same rules. The reality is that the version of the ActionScript language is closely coupled to the version of the Player. Player 10 includes a Flash VM that runs applications written in "AS10" and "AS9" and a Flash interpreter that runs applications written in "AS8" and earlier versions of the language. So why doesn't Adobe come clean and call the new version of the language "ActionSctript 10". Better still, since ActionScript's not a "scripting" language and hasn't been one for a long time now, why not just call it "Flash 10"?