It was difficult to learn what JavaFX exactly is from tech journalists beyond strategic dribble and comparisons to Silverlight and Apollo so I went searching.  From news articles, I thought it was just an announcement. Surprise! You can download the full source code and documentations, plugins for Eclipse and NetBeans, demos, and tutorials right now at OpenJFX.org website.
So what is JavaFX? This is what it looks like:

import javafx.ui.*;

Frame {

  title: "Hello World JavaFX"

  width: 20

  height: 50

  content: Label {

    text: "Hello World"

  }

  visible: true

}

      As you can see, JavaFX script (*.fx) is basically JSON-like version of XAML which can be either compiled into Java bytecode or run as is using a java-based JavaFX player. Since JavaFX is just java, it can run as applets in browsers, midlets on mobiles, and applications on WebStart.
    Hmm. Technology itself is not new but it seems to be more polished than I have come to expect from Sun although I think Sun will have trouble attracting developers without tools for non-programmer. I wish them luck nonetheless. 
 <em>Urgh. Looks like I need to fix page styling.</em>

Update: Sam Ruby has more on savory dynamic features of JavaFX scripting language which I neglected to mention.