I had bought the Magic mouse from Apple but I was not using it because of the lack of Expose buttons and middle mouse click.
Sure Apple will upgrade the mouse driver to support these but for now there is an application that adds these missing things to the new Magic Mouse…!!
It's the MagicPrefs application that you can download from here. The setup is very simple (you also see images for every action) and you can apply anything (spaces / dashboard / expose / apps) to tap or clicks.