Causing the biggest stir was a man who's barely out of boyhood, post-teen sensation Zac Efron, 21, playing adult in a sophisticated suit and sipping Grey Goose L'Orange with club soda.

"He was surrounded by an army of suits, management types," a party-goer tells Celebuzz.

Certainly less conservative than Zac's crowd was the "very pregnant" M.I.A., sporting serious baby bump in a body-hugging pink dress.

Keeping to the men, however, Leonardo DiCaprio, 34, turned up looking so handsome we're told Superman Returns heroine Kate Bosworth couldn't tear herself away from him.

"They were very close all night," a source spills. The duo took their party to hotspot Bardot in Hollywood afterward, were the intimate chatting continued.

There's been no indication that Bosworth, 25, has split from mainstay model boyfriend James Rousseau, 27.

Also at the GQ bash, Mad Men star John Hamm, 37, and costar January Jones, 30, Jason Statham, 36, Rosario Dawson, 29, and Megan Fox, 22.