Katy and Rob randomly crashed a total stranger's wedding rehearsal at San Ysidro ranch on Sunday. But an insider says there wasn't any PDA to speak of. And then on Memorial day she threw a barbecue at her new place in the Hollywood Hills, and among the guests? Her ex John Mayer. A friend says, "They seem to be back together, or back together-ish. They were very flirty and having a really good time together. They have good chemistry."