Its on the iPhone4 (which I do not have), but from what I can see, it appears to be a "gaming thing" that will allow you to tilt the iPad to get the mouse to move... I think.
I missed that part of Uncle Steve's presentation --
Well there is an accelerometer that kinda sorta does that, but this allows 6 axis sensing and for heavy duty gaming, this is apparently a real biggie!
And please don't ask me to explain what 6 axis sensing is... because I have no idea!
There is a very cool app that uses the accelerometer to track your 0-to-60 time in your car (because us Corvette guys really need to know that kind of stuff!) and it is amazingly accurate!
Currently the accelerometer lets the iPad detect being rotated in either direction - left and right. The 3-axis doohickey does left, right, up, down, and tilts at an angle - it senses the iPad's orientation in 3 dimensions rather than 2.
Imaging the mouse arrow in the middle of your iPad screen having the iPad laying horizontally in a table. If you want to move the arrow to the top left corner currently you have to tilt your iPad moving the top down until your arrow moves all the way to the top of the screen and then you move down the left side to get the arrow to the corner. With 3 axis all you do is to move down your left top corner.