Miley and Justin sitting in a tree?

The rumour mill is churning furiously at the thought of an undercover romance between teen stars Miley Cyrus and Justin Bieber.

Cyrus made a surprise appearance onstage during Justin Bieber’s sold-out show at Madison Square Garden in NYC on Tuesday night (August 31). The crowd went into a frenzy during their duet of Bieber’s song Overboard.

Justin Bieber recently mused about doing a remake of Grease. When asked about his number one choice of female co-star, he replied: "She can sing, dance and act. I would go for Miley Cyrus without hesitation."

The two were spotted dining together in May. At the time, Cyrus said the two were just working on a "side project" which was likely the NYC concert this week. And she was dating Australian actor Liam Hemsworth.

Now the Hannah Montana star is single and the gossip sites are looking for signals the two are more than just friends.