Here's a simple 80/20 solution for people with lon/lat tables of points wanting to do distance searches. It only works for places less than 60degrees north or south, and only for points. If you know your data is all more southerly than 60d, adjust the constant factor to make the index portion of the search more selective (and hence, efficient (faster)).

-- 
-- ST_DWithin_Sphere(lonlat-point, lonlat-point, radius-in-meters) returns boolean
--
-- Meters/Degree @ 60N: select 1/distance_sphere('POINT(0 60)','POINT(1 60)') = 1.79866403673916e-05
--
CREATE OR REPLACE FUNCTION ST_DWithin_Sphere(geometry, geometry, float8)
        RETURNS boolean
        AS '
            SELECT $1 && ST_Expand($2,$3 * 1.79866403673916e-05) 
               AND $2 && ST_Expand($1,$3 * 1.79866403673916e-05) 
               AND ST_Distance_Sphere($1, $2) < $3
        ' LANGUAGE 'SQL' IMMUTABLE;