Parts of Maemo are open source, and can be found at maemo.gitorious.org (off the top of my head). However, for whatever legal reasons, not all of Maemo is open source. Parts from Nokia are still closed, and drivers for things like video can't be opened by Nokia.

If you are interested in something more open than Maemo, you may want to look at MeeGo (http://www.meego.com) which has no closed source bits in its stack, unlike Maemo.