Things are getting serious!

After three months of sneaking around, Ashton Kutcher and Mila Kunis are happily packing on the PDA -- and aren't afraid of putting their budding romance on display.

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"Mila is throwing caution to the wind!" a source says of Kunis, 28, who held hands with new beau Kutcher, 34, while waiting in the ticket line at Hong Kong International Airport July 28 en route to a vacation in Bali. "She's having fun."

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Kunis' change of heart surprises pals, the insider tells Us Weekly, since up until now, "she downplayed the significance of the relationship to her friends because they think she's too good for Ashton."

The former That '70s Show costars' romantic getaway came two days after the duo cozied up at the wrap party for the Steve Jobs biopic -- starring Kutcher as the late Apple founder -- at the Duplex in West Hollywood.

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"They whispered and rubbed each other's thighs," says an eyewitness at the Jobs bash. "They snuggled all night!"

Despite their PDA-packed outings, Kutcher and Kunis are "not exclusive," counters another insider close to the couple, who are eager to keep things casual for the time being. "But they are hanging out and seeing where it's going."

This article originally appeared on Ashton Kutcher, Mila Kunis Take Secret Trip to Bali!