I don't know when this broke - maybe around the time Safari 4 was released? Anyway, Java WebStart stopped working. Downloading a .jnlp file and doubleclicking it brought up an editor (Dashcode) rather than the application launched via Java WebStart. I tried using Spotlight to find "Java WebStart" so I could manually launch the app. But nothing was found. WTF?
Apparently, the solution, of course, is to navigate in the Finder to /System/Library/CoreServices and click on Java WebStart.app - an entirely intuitive and obvious solution. This triggers some hidden magic to somehow restore access to JWS. Who knows. It works after doing this.