Now that iOS 4 is no longer NDA, I would like to know what Gyroscope has to offer over the Accelerometer for developers. Is there a difference in APIs? Other things?
A MEMs gyroscope is a rate of change device. As the device rotates in any its axis, you can see a change in rotation. An accelerometer only provides the force along the X,Y,and Z vectors, and cannot solve for "twist". By using both sensors, you can often implement what is referred to as a 6DOF (degrees of freedom) inertial system - or dead reckoning - allowing you to find the relative physical location of the device. (Note that all inertial systems drift, so its not stable in the long term).
In short: gyroscopes measure rotation, accelerometers measure translation.
There is a new API for reading the gyroscope.