The argument over whether Joba Chamberlain should be a starter or a reliever will never die down. The fact of the matter is that he will be a starter this year. It has been stated many times by Brian Cashman. But what happens down the road when Mariano retires? Will he shift to the bullpen and become the successor to Rivera? Nobody can predict the future, but it starts and interesting discussion.
Joba isn't going to close now, so why waste three years of his career pitching the 7th or 8th inning? Would the Yankees have moved Dave Righetti to the bullpen as a set up guy? Would the Red Sox have kept Papelbon in the bullpen for an 8th-inning role. In his prime, would the Atlanta Braves have moved John Smoltz from the rotation to make him a middle reliever? I think not.
Joba full time in the bullpen only makes sense at this point if he is going to close. And, unless Rivera's shoulder does not respond to off-season surgery, that isn't going to happen until 2011.
So, let him start. If he reaches his innings limit, or proves he can't handle it, move him back.
Joba can be a great closer. Later. For now, he can help the Yankees best by being a very good starter.