I fully expect that Apple will add a second button to their trackpads soon. However, I expect it will be just like the mighty mouse. Looking at the design of the current trackpads, there is no reason why the button cannot incorporate 2 seperate sensors. Besides I wish to have only mouse button personally (this is how I have my Mighty set up).
What would be more interesting would be if Apple were to move the buttons to being above the trackpad (close to the screen). I'm not an expert in ergonomics but this could "feel" better to use.
Actually on second thoughts, after emulating this approach for a few moments, it seems the button is bbest left where it is.
Apple Adding A Second Button To Laptops? (A lesson in Speculation)