Both former child stars, the actor and Oscar-winner Portman, also 31, have been "friends for years," another insider explained to Us, and he was spotted hanging out with the actress and her son Aleph, now 13 months, back in February.
Coincidentally, he dated Portman's Black Swan costar Mila Kunis for eight years before they split in late 2010.
Culkin was back in the blogosphere this past week after a tabloid claimed he was suffering from a very serious addiction to heroine. The unconfirmed tale comes five months after an early February snapshot of Culkin -- looking shockingly gaunt and grizzled -- caused similar concern among fans. "Macauley Culkin is in perfectly good health," his rep said at the time.
Another boldfaced name at Portman and Millepied's hush-hush bash was Ivanka Trump, the first source adds; the businesswoman and socialite, 30, Tweeted photos from Big Sur, and added breathlessly on Sunday afternoon, "2 weddings on opposite coasts in one weekend! Just landed in NY & am heading straight to ceremony number 2. A lot to celebrate this weekend!"
Portman and Frenchman Millepied, 35, met on the set of Black Swan, and announced their pregnancy and engagement in late 2010. They sported wedding bands at the Oscars of this year.