Exes Cameron Diaz and Justin Timberlake Get Close on The Set of 'Bad Teacher!'

Uh-oh. Do you think Jessica Biel should be worried? JT is working VERY closely with his ex-girlfriend Cameron Diaz - who he dated for four years!

Cameron Diaz was pretty bitter about her 2007 breakup with Justin Timberlake - so we were super shocked to hear that she was planning to work with him on the new comedy Bad Teacher! Even more surprising? We've learned that these two exes have been getting VERY chummy on-set!

"Cameron and Justin look like they're really enjoying each other's company!" one Bad Teacher insider told HollywoodLife.com EXCLUSIVELY. Cam and Justin even filmed a steamy love scene for the flick earlier this week in downtown Los Angeles! "There's a great vibe on-set," said the insider.

But does this happy shiny reconciliation mean bad things for Justin's present girlfriend, 28-year-old Jessica Biel? "Jessica hasn't visited the set since Justin has been filming," says the snitch.

But a second on-set source maintains that things between JT, 29, and Cameron, 39 - who filmed at Cole's Diner in Los Angeles March 9 - are strictly platonic and professional.

"They're having fun together, but it's strictly professional. It's a really small set, so they've been together basically the entire time that he's been shooting," says insider #2, adding that the two definitely don't seem to be rekindling old flames. "I'm not sure if they hang out afterwards, but they definitely leave separately when the day ends."

While we're sure Jessica is glad to hear that particular piece of news, we'd still hesitant about letting our man work with his ex. Wouldn't you?