The New York Times has an analysis this morning of the two late night kings, and it's not a very flattering forecast for either of their futures. The writer of the piece even implies it might be time for Letterman to step down.
It's seems a bit harsh and premature to me.
I don't watch Letterman every night anymore, but when I do, I'm still impressed with his show. Then again, I'm no longer in a demo that anyone cares about.