Sienna's Topless -- Surprise, Surprise

Sienna Miller, topless in all her glory, snuggled up to her new main man, musician Jamie Burke this past weekend in Tulum, Mexico.

Sienna, 25, and Jamie, 21, were all over each other. Jamie even whipped out his guitar and started serenading Sienna — Jude Law‘s ex. One onlooker said, “They were like a couple of lovestruck teenagers. They seem totally in love.”

The pair met at a party to celebrate her latest film Factory Girl at a New York hotel at the beginning of this month.

Jamie Burke has been romantically linked to the likes of Kate Moss, Lindsay Lohan, and er, Courtney Love.

Sienna flew back to Los Angeles yesterday and will be attending the Oscars this Sunday.