Christina Aguilera at Beverly Hills

Christina Aguilera got all dressed up for her birthday – and not in a gown.The singer celebrated her 28th birthday on Dec. 18 with a Clockwork Orange-themed bash at Beverly Hills' SLS Hotel along with husband Jordan Bratman and pals Nicole Richie and Joel Madden.Arriving by a private party van, the singer and her friends headed into SAAM, the private dining room inside the hotel's Bazaar