While it seemed like every celebrity ever was in Chicago yesterday for the taping of Oprah Winfrey's farewell spectacular, David Letterman wasn't one of them. As he explained in a rant on tonight's Late Show, Letterman—who "spent the last 12 to 15 years kissing up to Oprah"—didn't take the snub lightly. Add to that the fact that a bit he'd rehearsed with Steve Martin for today's CBS upfronts was scrapped at the last minute, and it's easy to see why Letterman was a bit more cranky than usual this evening.