I just tried a program called LOSS (LCARS Operating System Shell) 2005. It was pretty disappointing (though you can try it at http://www.lcarsdeveloper.com/downloads.php). There were three applications: Alarm Clock, Tik Tac Toe, and Command Prompt, the last of which did not work very well. I think this program has a long way to go, though what has been developed looks promising.
Update (01/18/2008): It’s been a long time since this post, but if you’re still looking for an LCARS GUI, there’s always that thing I discovered called LCARS24. That was months ago, but I’m in the habit of updating my old posts now.