It seems like the Mighty Mouse has been pretty much deemed a disappointment. The main reasons being:

• The scroll ball is a bit small and hard to use.
• It’s still really only one button.
• It’s wired.

Here’s a couple ideas for a scroll mouse that gives you lateral scrolling:
Touchpad mouse: Build a touchpad into the top of the mouse, and get rid of the wheel. Clicks remain clicks, but you can scroll by moving your finger across the touch pad. If engineered well, it should look like a normal mouse, just with a special surface where your fingers go. You can get the same kind of accuracy that you could get with two-fingered scrolling.
Trackball mouse: Instead of using the middle finger to do the scrolling, why not use the thumb, which on most mice isn’t doing anything. Make a design that looks somewhat like logitech’s stationary trackball design, except make it smaller and actually act like a mouse (where absolute positioning is done by moving it around). The thumb can then use a medium sized track ball to control scrolling up down and around. If people can do it well enough for moving the mouse around, then it should be more than adequate for scrolling
I personally like the first idea better than the second one. But it seems slightly difficult to build. Can they build trackpad’s on curved surfaces yet?