She kissed John Mayer, and she liked it! After a very public split from ex-husband Russell Brand, Katy Perry took up with another infamous playboy at the Chateau Marmont in July. Since then, the couple has broken up and gotten back together a handful of times, and appeared to be on-again at a L.A. costume party where Perry celebrated her 28th birthday with her sometimes boyfriend. She "believes she can change" him, says a source. "She doesn't want to be just another one of his conquests." The couple also celebrated Mayer's 35th that same month at Freeman's in New York City.