Following a Person In Real Time Using RFID
I haven't done this, so this is just thinking out loud, but I'd try coded IR first.  The person would wear a belt or something with IR LEDs are various angles.

The cart would have two sets of receivers on it arranged so that each receiver can determine a rough idea of the direction it received the coded IR transmission from.  This could be accomplished, for example, with a few IR detectors in each receiver, with each detector having a narrow reception angle.  The two receivers would be spaced far enough apart so that when the cart got close enough to the person the parallax would be great enough to reliably discern it from inifinite distance.

If the same IR message with the right code isn't received from both receivers at the same time, or there is significant intensity difference between the two, the reception is ignored like it never happened.  The transmitter would send often enough so that missing a few individual receptions causes no real harm.