After Jimmy Kimmel did his show dressed as Jay Leno the other day, Jay invited him to appear on the "10 at 10" segment of The Jay Leno Show. Unfortunately the NBC provided embeddable clip currently only has the first question. And you really need to watch the whole thing. Gawker has the full segment (second video at that link).
Though it wasn't surprising to me to see Kimmel giving Leno the business, I was surprised he went quite as far with it as he did.