It was a great move by Letterman/CBS & Leno/NBC to run the commercial during the Super Bowl.
The ad was the funniest aired during the Super Bowl, and was one of the funniest things Leno has down in sometime. Inside baseball, sure -- my kids didn't get it -- but they're not the target audience for late night TV, yet. It showed they both could laugh at themselves, and it generated buzz, not a bad thing for either show. It won't stop the audience erosion that all network TV is experiencing, but it certainly gives adults a reason to talk about Letterman, Leno and their shows.
The real question is: what does Oprah get from appearing with Letterman in a second Letterman/Super Bowl commercial (as she did in 2007)? I'd say funny is funny, and that doesn't hurt.