Go to the list settings page. Right click on "Title, description and navigation" and copy the URL. Paste that into notepad and copy everything after "List=" in the string. That's your URL Encoded GUID for the list. All you need to do is decode it. I found this php page that does it for you so you don't have to whip up a quick program to do it. I'm sure there are zillions of others around.
Or maybe there is a URL decoder somewhere in the MS stack that I'm not aware of. Anyone? Anyone?