Sporting a blonde bob and a short black dress that showed off her newly svelte shape, the new mom – whose second son, Jayden James Federline, was born Sept. 12 – surprised the host by walking onstage behind him as he was sitting at his desk.
"Oh, my god," he exclaimed, standing up to shake her hand and plant a kiss behind her ear. "Hi, how are you? Nice to see you. Oh, you look wonderful."
"Thank you so much," she said. "I'm in town, and I'm just such a huge fan. I had to stop by."
"That's very nice of you to say," said Letterman. "That means so much to me."
Said Spears: "Uh, no, I wasn't talking about you. I was talking about Will Ferrell."