I just discovered this word today. I love it. Believe it or not, the term actually came in a conversation about UI guidelines. I proposed the idea of heroelements--elements which motivate the functionality or purpose of the app. Everett, the writer I was working with called these moments in the app the app's McGuffins.
Okay, so I'm not sure that it's a perfect analog, at least not the way I originally understood the analogy. It's a great word though, and there application to UI. A user experience is motivated by a task which is probably made up of a series of steps. Think of these steps as the story of the app. There are elements in the UI which motivate that story, even if they don't directly contribute to it, the McGuffins.
Here's an example: the desaturation of the desktop when you change themes in XP. Windows could just freeze up for a moment or give you an hourglass. The desaturated desktop, however, tells the story in a way that the hourglass can't. It's a "plot element...that catches the viewer's attention or drives the logic or action of the plot."
Is this a stretch of the term? Probably.