Miley Cyrus's wedding shocker!

No one could blame Liam Hemsworth for wanting to escape the noisy crowd at a Chateau Marmont bash in LA. But jaws dropped when onlookers realized the beautiful blonde who slipped into a waiting car with the Hunger Games star on Feb. 23 wasn't his fiancee, Miley Cyrus- it was Mad Men bombshell January Jones. "It was a complete shock," says an observer who saw the two speed off together." Just Liam and January in the back of the car." That was just the beginning, after the hotel, the pair stopped at a private house party. "January was clinging to him," says a witness. "Liam was saying, 'We can't do this here.' They went into the hallway and started making out!" The scene would surely have devastated Miley, 20. "Miley's always felt Liam is so talented, so hunky that a woman could easily steal him," says a longtime friend of hers. "That insecurity is always with her." And along came January. "She kept saying to Liam, "You're so handsome," says the party witness. But what about Liam's very public commitment to Miley? "Whatever January wants, January gets," warns a producer who has worked with the actress, 35. "She doesn't care if a man is with someone else or not." The pal insists that Miley and Liam, 23, are still engaged, and the singer was sporting her 3.5 carat engagement ring as she headed into a Glendale, Calif., recording studio on March 1st. But the couple rarely been spotted together and Liam left the house they share to go to Australia to see his family on March 2nd. Then he'll be in Asia for work. "Miley's freaking out," says another friend. "She gets super jealous and thinks January is after Liam. Miley's been texting him nonstop about it. He says nothing happened." For now, at least, Miley is choosing to believe her man. "She's saying she trusts him," the second friend says. "He's telling her he loves her. But if she found out he cheated, the engagement would be over."