Justin Bieber performed at Madison Square Garden last night - which was being filmed for the tween star's upcoming 3-D biopic - and the crowd went wild when Miley Cyrus took the stage!

The 17-year-old single singer joined Justin for "Overboard" and, we can't help but wonder, just how close are these two? Their chemistry is undeniable!

Heck, Justin even wants Miley to be his co-star in the Grease remake... if they ever do it.