BIND servers use round-trip-time to select the fastest NS from a set of NSs, automatically accomplishing your objective. Asian queriers "should" see asian NSs as having lower RTT and will home on to them, and N.A. queriers would home one to N.A. NS.
The original question was about serving *web* content from the closest server.
-- Rob Szarka, Bizgrok Inc. http://bizgrok.com/
