Here are my tests:

// Radius of the earth: 3959.8728
// Distance to Maplewood, NJ (in radians): 0.0036998565637149016
// Distance to Maplewood, NJ (in miles 0.0036998565637149016 * 3959.8728): 14.65

// Does not return Maplewood
db.example.find( { loc: { $nearSphere: [ -73.99171, 40.738868 ], $maxDistance: 0.0036998565637149016 }});

// To get a list of all places (with distance in radians):
db.runCommand(

{ geoNear : "example" , near : [-73.99171,40.738868], spherical: true }

);

// This should include Maplewood, does not.
db.runCommand(

{ geoNear : "example" , near : [-73.99171,40.738868], maxDistance : 0.0036998565637149016, spherical: true }

);

Venues:

geoDoc =

{"name" : '10gen Office', loc: [ -73.99171, 40.738868 ] }

db.example.insert(geoDoc);

geoDoc =

{"name" : 'Maplewood, NJ', loc: [ -74.2713, 40.73137 ] }

db.example.insert(geoDoc);