For eight nights beginning just before the 4th of July, Rachel Maddow, the host of a program on Air America, the liberal talk-radio network, served as a substitute for the vacationing Keith Olbermann. "At some point, I don't know when, she should have a show," said Phil Griffin, hours before he was promoted on Wed to president of MSNBC. "She's on the short list. It's a very short list. She's at the top."