Althouse has this cryptic post, which prompted a Commenter on her blog to utter the phrase in the subject line.
Which reminds me of a joke I heard a comedian (do we still use the term "comedienne"?) tell once. She was observing that with the song "You're So Vain" Carly Simon was really putting Warren Beatty in a bit of pickle. Once the song comes out, he has two choices--either he can admit that he is, in fact, so vain that he probably "thought the song was about him." And if he doesn't admit it, then Carly is going to write a follow-up song for the next album, "You're so stupid, you didn't even realize the last song was about you." Either way, he's stuck.
I had always understood that this song was about Beatty, but as noted in the Comments, apparently Carly Simon has never actually verified this. According to this, she has officially denied that it was James Taylor.