I see from these that there is no special support intended for JavaFX 2.0 in IntelliJ:
JavaFX 2.0 was released a few days ago and I think it is worth you having a look at it and possibly reconsidering: http://download.oracle.com/javafx/
JavaFX 1.0 was a miserable failure and a terrible idea. JFX2 is a completly new approach and is much more likely to make an impact and stay around (at the very least it should gradually replace Swing, but should also make some inroads into web against Flash, etc). The most important thing is that the horrible scripting language has been ditched for pure Java. What we're left with is a Java API for building powerful desktop GUIs (and applet-like programs) with animations, styling, special effects and styling. A long overdue successor to Swing. There is also talk of it being supported on mobile devices such as Android and iOS.
Since the core library is all Java now, there's not much special support needed for this, but there are two areas that IntelliJ could add value:
Ideally the support for both of these would include smart referencing, where attributes referenced in the FXML and not specified in the controller are highlighted in red, and CSS styles not used are greyed out, etc.
I personally don't see any real need to have an IDEA specific UI builder, but there is an official SceneBuilder under development by the JFX team (due out in a couple of months). You may want to look at including this into IDEA as a plugin or something, but this is a bonus extra and probably not something I personally would use.
As a long time IntelliJ user (since version 3 I think), I would definitely like to see support for this added in. At the very least keep an eye on the JFX space and watch for developments - don't rule it out because of the shamozal that JFX1.x was.