Every book I've ever seen gives a different number. According to a
href="http://whistlepig.cs.indiana.edu:31415/q700/node2.html"this/a
paper at Indiana University, it's 1500 Hz. As memory serves from
acoustics a few years back, that is more or less correct. Somebody
correct me if I'm mistaken...

More on localization for anyone who's interested:
http://en.wikipedia.org/wiki/Sound_localization

Mike