I am building a robot with the same goals - follow a person around.
Independently of this thread or any of the links in it, I decided to use 2 ultrasonic mics on the robot and an ultrasonic driver worn by the person - great minds think alike! The mics will be mounted on a rotating head to allow the robot to re-acquire the signal even if the robot is not facing in the right direction.
For obstacle avoidance I am using 3 ultrasonic sensor arrays (left, forward, right).
Currently I have an obstacle avoidance sketch that has no following capability to prove out talking with the obstacle sensors and controlling the drive motors. I need to build a mic amplifying circuit and work out the math to determine the beacon direction based on the mic inputs.