Personally, out of the frameworks you listed, I've developed applications using Swing, SWT/JFace, and played around with XUL some. This is actually the first I've heard of Spring's RCP; I'll have to check it out.
Honestly, leaving Spring RCP aside simply because I'm not familiar enough with it, each of the others has enough major backing and significant projects behind them that I would be very surprised to see them go away any time soon. From a longevity standpoint, I'd imagine you'd be fine using any of them, so a decision would need to be made based on the other requirements of your particular project.
Stephen Holder<br />Co-author of <a href="http://www.amazon.com/exec/obidos/ASIN/1932394273/ref=jranch-20" target="_blank" rel="nofollow"><i>SWT/JFace in Action</i></a>