YMMV, but I’m very happy with Apple’s Magic Mouse – the absence of a scroll wheel or button is bliss, and the ergonomics, while not as good as some mice by Logitech, are quite good. What I do miss however is the third button, and having other commands via alternative gestures. TUAW even posted an article earlier calling the Magic Mouse a “dog”, in part, due to the lack of a middle button.

Thanks to ifixit.com, we can appreciate that the underside of the Magic Mouse sports a grid of 10 x 13 sensor pads from the logo towards the top of the mouse, and two final rows at the very top, one with 8 sensors and the other with 6:

This gives the mouse the potential to detect one or two fingers placed on it, and the individual motion of each finger, anywhere on the mouse’s usable surface. So, I propose the following gestures. To middle-click (third button), simply place two fingers on the mouse, and click with both at the same time, thus:

To open Exposé or Spaces, place the right finger on the mouse, and scroll up or down with the left finger:

It’s actually quite ergonomic, try it on your mouse, and if you like it, send the suggestion to Apple – if anyone finds a form specific to the Magic Mouse, let me know, otherwise the link points to iMac feedback (as it comes with one). And yes, the images above are taken from Apple’s page and badly photoshopped, my apologies!