A repost of the code for ST_Buffer_Meters from this great post: http://www.gistutor.com/postgresqlpostgis/6-advanced-postgresqlpostgis-tutorials/58-postgis-buffer-latlong-and-other-projections-using-meters-units-custom-stbuffermeters-function.html

st_buffer_meters.sql

/* Function: ST_Buffer_Meters(geometry, double precision)
DROP FUNCTION ST_Buffer_Meters(geometry, double precision);
Usage: SELECT ST_Buffer_Meters(the_geom, num_meters) FROM sometable; */
 
 
CREATE OR REPLACE FUNCTION ST_Buffer_Meters(geometry, double precision)
RETURNS geometry AS
$BODY$
DECLARE
orig_srid int;
utm_srid int;
 
BEGIN
orig_srid:= ST_SRID($1);
utm_srid:= utmzone(ST_Centroid($1));
 
RETURN ST_transform(ST_Buffer(ST_transform($1, utm_srid), $2), orig_srid);
END;
$BODY$ LANGUAGE 'plpgsql' IMMUTABLE
COST 100;