Jessica's New Romance
08/29/2006 AT 12:00 PM EDT
Take it from Daisy: Mayer, 28, is officially in with Simpson, 26. And the bubbly pop star – whose promotional tour for her new album, A Public Affair, has been slowed by a bruised vocal cord – can hardly contain her delight at falling for the scruffy-haired singer-songwriter with a sensitive streak. "She's head over heels," says the source close to her. "She's telling friends, 'I'm in love.' "