Karl Milan / Splash News 1 / 8
Karl Milan / Splash News 1 / 8

By the end of Thursday night, Miley Cyrus' exhausted facial muscles must have been begging for a break. The starlet, 17, smiled nonstop as she made her red carpet debut with boyfriend Liam Hemsworth at the Los Angeles premiere of their three-hanky romance, "The Last Song."

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Miley, looking lovely and age-appropriate in a one-shouldered pleathery black dress, was repeatedly snapped gazing adoringly at her Aussie beau as they posed arm-in-arm (click through the photos at left for more looks of love). Alas, he apparently didn't stay glued to her side for long.

"I lost him so he's not being the best boyfriend right now," Cyrus joked to E! News. "The whole deal was we were going to be walking together, but then I lost him first thing because he started saying hello to everyone."

RELATED VIDEO: Miley and Liam at "The Last Song" premiere

Liam's disappearing act came soon after Miley gushed to Ellen DeGeneres about the chivalrous display he put on when they first met at his audition for "The Last Song."

His gentlemanly gesture: He opened a door for her. (Anyone else feel the bar needs to be raised a little on what young women expect from a guy? Yeah, us, too.)

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"I was like, I have been in L.A. for three years and I don't think any guy has actually opened the door for me," Miley bubbled to Ellen in an interview set to air on March 31, the same day the film hits theaters. "I was like, wow, that is super impressive. I actually turned to the director and said, 'He's got the job.' He's hot and he opened the door. Excellent."

Hemsworth, 20, however, could be regretting his courteousness.

"There's so much pressure for me to be opening doors for everyone now," he laughs to E!. "It's just too much pressure."

As for what drew Liam to the raspy-voiced teen queen, whom he began dating last summer on the Georgia set of the movie, she "makes me laugh," he shares with Radar Online.

For Miley, it was a case of opposites attracting.

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"He's a 6-foot-4-inch Australian surfer and I'm 5-foot-4-inches from Tennessee, and I couldn't surf if someone gave me ten million dollars so we couldn't be more opposite," she rapid-fires to the London Telegraph. "But I went to Australia to visit him and his family and I was like, 'Omigosh, this looks like Nashville.' There were like cows everywhere. I am like, 'How are you from the opposite side of the planet and your world looks just like mine?'"

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Hemsworth also has the approval of Miley's dad Billy Ray, who, come to think of it, also approved of her last boyfriend, older underwear model Justin Gaston, so maybe he's not all that picky.

"My dad's just happy that I have something going on in my life with someone normal," explains Miley. "No psychos, so that's good."

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