Is Marc Jacobs Engaged?Marc Jacobs may have given a Cartier engagement ring to his on-again, off-again boyfriend, Jason Preston. Tyra Banks dropped her manager, either because he was a prima donna or because her investment-banker boyfriend told her to. Britney Spears backed out of recording a Timbaland-produced duet with Justin Timberlake. It’s unclear why. No cameras or cars are allowed at the fund-raiser Oprah is throwing for Barack Obama at her California ranch, which is expected to draw George Clooney, Halle Berry, and Jamie Foxx. Harvey Weinstein is offering $100,000 to anyone who can identify the Upper East Side mom who inspired The Nanny Diaries. (Some speculate it’s Preppy Handbook author Lisa Birnbach.) Marc Ecko’s CEO threw $500 in cash around during a company-sponsored booze cruise. Norman Reedus, Helena Christensen’s baby daddy, is making a movie in which Richard Nixon sleeps with a hooker and then kills her. U.N. Secretary General Ban Ki Moon dined at Le Cirque with two tables’ worth of security guards.