Used a A1301 Hall Effect sensor in this sketch
http://www.arduino.cc/playground/Code/HallEffect
Download the US5881 datasheet from
http://www.melexis.com/Assets/US5881_DataSheet_4824.aspx
and you will see it's output cannot be used to measure the strength of the magnetic field. This sensor will switch OUT state at two threshold values.
No experience with the MPF102