I want to clear a bunch of things up in this thread.
Accelerometers are 'bout the only thing you can use for
velocity acceleration measurements.
Fixed that for you. I'm not usually a grammar cop, this one just bugged me for some reason. Accelerometers only measure lateral acceleration.
Ok, here's the mini lesson that should clear some things up.
Accelerometers measure acceleration, not velocity. They can be used for orientation sensing since gravity is constant, but that's assuming there's no acceleration in other vectors since they can only measure the sum of the forces. For example, a accelerometer upright with a secondary lateral acceleration in the x direction (assuming z direction has vector due to gravity) will have similar output to a sensor oriented slightly so that the gravity vector has a x and z term. The only difference would be magnitude. Also, they are very noisy.
Gyroscopes measure angular velocity, from which angular position can be derived through integration. They are fairly sensitive but have a single weakness of drift. Since they have to be sampled discretely, any velocity data outside the average of two subsequent samples is lost, and causes the derived position to 'drift'. For example, if you rely entirely on gyros, after a few minutes, looking forward using your helmet won't be looking forward in the game anymore. The more you play, the worse the drift gets.
A compass measures absolute angular position, but only in the xy plane. It is a slow and noisy sensor, but never drifts. North is always north (well, not always, but that's another lesson).
For full 3D inertial measuring, a common practice is to combine the fast and accurate sampling of gyros, the absolute reference of a compass for XY vector, and accelerometers to measure gravity for the absolute reference in the other two planes. If you miss any one of these three pieces, you will not have a fairly accurate measuring sensor.
Wii gets around using a compass by using an IR led and camera as the absolute reference for angular position, and uses gyros and accelerometers in-between IR reads.