The Letterman 'Balls' clip has generated lots of interesting comment. I haven't changed my view that it's OK to borrow though. Especially if you're building on an idea, improving it, making it your own somehow.
(Having said that, I do take the point about acknowledging your sources. You look a bit mean if you don't. Like the creators of Honda 'Cog' always claiming their inspiration was the game 'Mousetrap' rather than the short film by Fischli & Weiss.)
But no debate about this one, surely. Compare the 2007 D&AD Student Awards Call For Entry with the promotional materials for the 4th Annual One Show Exhibition at the Ad Museum of Tokyo from 2005.
Listen closely, graphic design consultancy Lippa Pearce. You can steal from a movie. You can steal from a promo. But you can't steal from another ad. And that's just the way it is.