I'm starting a countdown - apropos of nothing other than the fact that one is required to fill up a TV blog with ruminations about TV - to the day when George Clooney finally says "yes" to a final appearance on "ER." He will do it. I am certain. The only thing I can't quite predict is the exact day when his PR man, Stan Rosenfield, comes out with the statement saying something to the effect that GC will in fact make one final cameo ... ... on the show that made him world-famous, started his career and got him to where he is right now. (Other than innate smarts, talent and a good career barometer, which he does have.) Of course, Clooney could also argue that "ER" wouldn't be around without him, and he might have a point, but not quite as convincing as the one just made - that GC would be an eternal ingrate if he doesn't make one single last farewell appearance , for the sake of the show, his old boss, John Wells, and to those fans who have actually stuck with the series all these years. So the countdown begins. Now, you're thinking, "come on Gay. There has to be a reason you're doing this now." And I do have a reason: Clooney led off the terrific "American Masters" series on Warner Bros. last week. His was the very first mug on camera. The very first guy this film by Richard Corliss chose to pick to reveal to the world as a symbol of this empire. Why Clooney? In part - admittedly very small part - that he was the biggest name outt ...