An added bonus with this plugin is the distribution of the ux-all package (it comes in the package with Ext JS) as well as a slightly modified version one of the examples - the Desktop application.
So what I did to the Desktop example was to externalize the creation of desktop icons (it was really stupid to code all the icons directly in HTML) as well as the inclusion of ScriptLoader utility that allows to load additional scripts on demand.
If you'd like to give it a try there's a load of information on the plugin's page on how to create a desktop-like application in under 5 minutes :)
And if you'd like to get the sources here's the git repo with them: http://dev.aplaline.com/git/grails-extjs.git
The sources also contain some documentation (which is basically the one from plugin's page but dissected into smaller chunks for easier navigation). To generate the final version run grails doc in the main plugin folder and in a matter of seconds target/docs you'll find a ready to browse version of the documentation. I might put it online some day :)