I've been playing with Microsoft's script encoder [msdn.microsoft.com]. It does indeed encode (not encrypt) js scripts. They are fairly easy to decode, given that there are now numerous sites offering decoder scripts. Still, it'll keep your scripts beyond the reach of the average web-hanger.
Use of this limits your js to IE5 browsers. My assumption is it will never work on NS (it doesn't on NS4) nor Opera, but I don't really know.