Dave's Girl

One man's chat-show guest is another man's wife, but that doesn't stop David Letterman from flirting. For a while, it was Connie Chung. Now it's frequent Late Show invitee Phoebe Cates. Does her husband, Kevin Kline, have a problem with that? ''Noooo, I really think it's funny,'' he says wryly. ''Yeah, funny.'' Kline says he knows that Dave's on-air dalliance with Cates — a longtime friend of Late Show producer Bob Morton — is just a running gag. ''Dave has all these little slams for me,'' says Kline. Meanwhile, Morton says Kline has nothing to fear. ''Dave is very fond of Phoebe,'' he says, ''but it's a professional relationship.'' Kline's not worried. ''It's all show,'' he says. Pause. ''Isn't it?''