Justin Timberlake and best friend Trace Ayala debuted the Spring/Summer ’07 line of their William Rast clothing imprint at Social Hollywood last night as part of L.A.’s Fashion Week. Before the pair’s denim line made its runway debut, an interview with the two played over a loudspeaker in which they spoke about sexy being an attitude.

Accordingly, the typically staid runway show was given an energetic makeover, with a couple of lively dance numbers, one showcasing four breakdancers. Timberlake and Ayala then strutted down the runway to take their bows and were presented with bouquets.

But J.T. was not done for the night. An hour later the lucky 600 or so guests — including Cameron Diaz, Paris and Nikki Hilton, Michelle Trachtenberg, Eve, Timberlake’s former *NSYNC mates Lance Bass and JC Chasez and many more — were treated to an intimate performance in an adjacent room, as the host led a singalong version of ’Cry Me a River’ and welcomed close friend will.i.am onstage for a dynamic ’Damn Girl.’ Other guest performers included Chasez and Maroon 5’s Adam Levine. Tres chic!