Accelerometers can only detect orientation relative to gravity, gyros can detect it relative to themselves.

This is the same thing that Nintendo added to their controllers a few months ago.

For example I wrote an app that calculates Horsepower based on weight and acceleration, however it's hard to determine if detected acceleration is due to the vehicle or gravity (if the device tips or tilts during the test). By having the gyros available, I can detect and compensate for the error-inducing influence of gravity.

If you have ever used a wii remote, it's like the Wii Motion Plus add-on (see: http://en.wikipedia.org/wiki/Wii_MotionPlus)

In short, the accelerometer detects movement the gyroscope detects exact direction.

My guess is that it will improve the response of the compass. Really handy for navigation.

I think the accelerator can only pick up pitch, yaw and roll while the gyroscope might also be able to pick up left/right and up/down (or is it the other way around?). Not sure about that, though.

The G1 has one of these in. Not sure about other Android based phones, but I'm assuming it's going to be fairly common if it was available in the original Android phone...