quaternion & rotation matrix help:

I need help with part of an iphone project.

i would like the phone to react differently depending on where the user is holding it relevant to its starting position.
say the app starts and if there is a "hot spot" behind the user and in the air.
the user has to find this spot to receive a certain reaction (sound, vibration, etc.)
the location of the "hot spot" should not change if the phone is rotated, shifted, etc.
the user should be able to walk in a circle around the hot spot.
it needs to combine rotational orientation with spatial gestures to make an estimate of where it actually is within a 3d space.
it seems that the location will be a calculation of the force from the accelerometer with phone orientation information.
i understand this estimate will not be extremely accurate.

the final result will be the phone returning a stream of current coordinates within a 3d space (x,y,z) where the origin is constant.