Wheels on the bus go round and round... *sigh* this looping discussion has gotten so old that it isn't even mildly amusing anymore.
QML components, or libmeegotouch (the things - minus the theme assets - that can make Harmattan apps) are both open source. The X11 compositor - mcompositor (hildon-desktop's smaller brother). meegotouch-home (the home screen for Harmattan). There's plenty more in terms of middleware once you start going looking, too - things like QMF (for email/messaging), ... The main parts that *aren't* open are the plugins that do the logic for the homescreen/app switcher (newsfeed screen, launcher screen, etc) and the applications. Almost all libraries underneath those - that weren't directly UX related - are open. Take a look on meego.gitorious.org sometime.