Proximity sensing

Hello all, I'm trying to figure out a way to sense the proximity of a specific object/device 1-2 meters away. The idea is to trigger certain events when a user carrying a device is approaching. I've been thinking about creating a magnetic field pulsing on and off and having the Arduino looking for this with a hall effect sensor, but I don't know if the field would be powerful enough to affect the sensor, or the sensor sensitive enough. Can anyone provide any pointers on how I might solve this? Thanks, /Thomas