I came across this piece in the New York Times today: http://www.nytimes.com/2011/04/11/arts/design/woody-woodpecker-and-shamus-culhanes-animation.html
I’ve long known about animators inserting cameos in their work. But this was quite new. And the idea of inserting different kinds of art as transitional backgrounds or tonal matte paintings is a great one. I’ll have to think on that a bit!