One combination I've been itching to try is Griffon with a RESTful Grails app on the back end. About a year ago I was working on a project that we "thought" needed to be a desktop app and I looked into a lot of different possibilities and Swing with a handful of helpful API's seemed to work best. We've since then gone full web based, however.
I find myself using GWT for web based UI. For me, the power of GWT is swing like coding. Lot of GWT derived flavors exist (like smart gwt) which can be easily adapted. You can also easily plugin to any other google api like google charts or google app engine.