Today Victoria Beckham tweeted it. But is this reverse form (as opposed to 北京，你好) correct?
Both are correct!! 你好，北京 is more like a casual greeting, e.g. Hello World! :D 北京，你好 is more like a greeting with a touch of respect, because you address the name first. Examples are, 张老师，你好 (Teacher Zhang, hello).
"你好，北京" is usually said by one come to Beijing the first time becaurse Beijing is a new friend to him. However, "北京，你好" is usually used by one has lived in Beijing for many years and he treats Beijing as his old friend.