As of February 2013, JavaFX is partially open source and should become almost fully open source over the next few months. The source is hosted as part of the open-jfx project that you linked in your question.
The manifest of the tip of the JavaFX development branch for Java 8 is: http://hg.openjdk.java.net/openjfx/8/master/rt/file/tip
The same page has a zip link, which allows you to easily download the available JavaFX source as a zip file.
The JavaFX source code is not included in the jdk src.zip file currently, but should be eventually before Java 8 is released.
A February 2013 update for JavaFX OpenSource status has been published by the JavaFX development lead.
In short, it is not open source in that it is free to change & distribute for any purpose. JavaFX is licensed for internal use, excluding what they call commercial features.
Use of the Commercial Features for any commercial or production purpose requires a separate license from Oracle.
You can get JavaFX from here.