As it did on Wednesday, the audience for Thursday night's edition of American Idol shrank significantly, down a whopping 27 percent from the season's second episode a year ago. It Nevertheless remained the most-watched show of the night, attracting 17.7 million viewers. But a new episode of CBS's The Big Bang Theory was breathing down its neck, with 15.7 million total viewers and about the same number of viewers 18-49 -- perhaps a few more. Also on CBS, the second week of the controversial comedy Rob! was down an expected 13 percent from its debut a week earlier but nevertheless attracted 11.7 million viewers. On the other hand, NBC's 30 Rock drew its lowest ratings ever -- just 4 million viewers tuned in.