Miley Cyrus and Liam Hemsworth are apparently back together yet again -- but don't expect it to stick.

Us Weekly reports that the oft-estranged but engaged couple are back on. Spies say Cyrus had her lips on something other than a blunt for once -- they were on her fiance's face during a party at their pad on May 10.

"She was kissing him," a witness said. "They're definitely back on."

Still, sources close to the couple don't advise anyone to buy bridal shower gifts just yet. Or ever.

Hemsworth's big brothers, 'Thor' actor (and living Adonis) Chris and Luke reportedly want the pair to end their yo-yo relationship permanently because, well, they're sort of sick of hearing about all the ups and downs. Sources said the bros staged a mini-intervention for Liam, explaining simply, "They want him to end it for good."

Added a pal, "They break up, get back together, repeat. They will eventually burn out."