"People near me" query

Imagine an Orkut-like site. Suppose we have 'person' table of 100k
people. About 75% of these people fill in their location
(City/State/Country) information. We also have a 'city' table containing
list of cities with their state & country and each city's
latitude/longitude. Assume all people's location is registered in the
'city' table.

How does one design a database to be able to process "Show me people
that live no farther than 250 miles from where I live" quickly? I can do
"Show me people that live within (A-X to A+X) latitude and (B-X to B+X)
longitude" though. (Where A and B is the latitude and longitude [of the
person], and X is some numeric value.

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dave