Head on over to
http://www.macosrumors.com/20050317.php
They have a lot more detail on the design of this mouse. Essentially, it sounds like it will look exactly like the current mouse... by the sounds of it, you won't be able to tell the new 'two button' mouse from the current mouse. It will act as a one button mouse by default, but you can turn on right clicking in the mouse control panel. And it will have scrolling via a touch sensitive surface.
So my guess is, there is physically just one button/switch inside the mouse, but the touch-sensitive surface, when enabled, will sense which side of the mouse you are touching when you click. Scrolling will be accomplished, presumably, by stroking the top of the mouse.
Cool innovation, but the fact that the ergonomics (shape/size) are not changing and it's still one massive button means, in my books, it's still completely worthless... and probably even worse than the current mouse. For the love of Pete Apple... just give us a mouse with normal friggin' buttons!!!!!!!!!! And a decent size/shape!
This is yet another example of the cart pushing the horse. In this case, the idea of using a touch sensitive surface is driving the design instead of HUMAN FACTORS driving the design. Apple mouse design is all about cool looks or cool technology... human factors always takes a back seat. That's just bad design.
One thing I DO like is that with it being one-button by default, apps will still be able to work with one button. I agree that the idea of things being only available via right-click is bad UI.