Hi,
just a quick note on some progress we do with FengGUI: We dropped another dev-release today that finally includes the full function set for XML theme loading. Although minor things may still change, the general framework is now in place. The nice thing about the framework is that generally you can not only read-in styles but you can serialize (i.e. read and write) complete GUIs including layouts, positions, sizes, items and such. However the latter still needs some polishing. 
We received several nice reports from people who successfully use FengGUI on top of jME so that we are encouraged to enable FengGUI to run on other scene renderes as well (e.g. Xith). The Xith support is actually the next thing we are going to work on.
Although we dont want FengGUI to grow much bigger in terms of code and complexity, we are planing to add a little effect mechanism that allows (among other things) to map windows and containers on textures in the background to open the door to all these fancy effects we know from MacOS. Composing GUIs is also planned to become easier especially with regard to HUD design. I keep you posted.
You can download the latest FengGUI here http://user.cs.tu-berlin.de/~schabby/FengGUI/.
Johannes