Via email from Mark Reinhold, Principal Engineer at Sun Microsystems -- it appears that JOpt Simple is being used for some of the command line tools in the OpenJDK:

"I thought you might be interested to know that we're using your jopt-simple library in the open-source Java Development Kit. Thanks for writing such a nice little library!  It's far cleaner than any of the other alternatives out there."

Great feedback! If you need to parse command line options with a minimum of fuss, please check out JOpt Simple 3.0. I look forward to even more widespread adoption of the library.