Justin Timberlake and Cameron Diaz Left Together On Saturday

Timberlake had joined director Martin Scorsese and about eight other people at a table inside the hotel's exclusive bar. Timberlake, in a striped coat, chatted briefly with Scorsese (who directed Diaz in Gangs of New York) and others at the table.

Then, at about 1 a.m., Diaz arrived. "Cameron never joined their table," says the observer. "She just showed up and Justin got up and started talking to her. They were being very discreet.

"They weren't kissing or holding hands or anything, but they were being very friendly," the observer adds.

The pair, who broke up last month after nearly four years together, then walked out the front door together a half-hour later. Says the onlooker:

"When I first saw this girl, I thought, 'That's not Scarlett Johansson or Jessica Biel.' Then it hit me – it was Cameron! She was low-key, her hair was dark and pulled back."