distance ranging and detection

Current project needs a method of detecting the presence of someone in a room who is approaching an interactive sculpture.

First I need to detect that someone is in the room (which activates a response from the sculpture).

Then I need to measure the distance of the observer from the sculpture, as the closer someone gets the different responses will be realized.
This distance detection could be either from an ultrasonic sensor
(from Parallax), or the low cost infra-red detectors from Sharp which are available at http://www.acroname.com/robotics/parts/R48-IR12.html