I'm interested in UI testing a client only Java application. What is the most popular framework for doing so? What is your recommended framework? Also, why doesn't Selenium do this (or does it)? It seems to me that anything that can test a web app should be able to test a windows app.
Try FEST framework. This is what was previously known as Abbot, if I'm not mistaken. I use this for automated testing and it seems to be very simple and convenient. Simple things are made easy and complex things are not a rocket science there. I considered UISpecj4j, but it didn't suite me, because there is no technical possibility to test Drag'n'Drop while it's a must. Besides it's quite difficult to develop tests when you don't actually see what's happening (UISpec4J uses fake, invisible look'n'feel)
The only thing is making troubles in FEST (also, by design) - it grabs a mouse and doesn't let you do anything while it running a test.