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

utmzone.sql

/* Function: utmzone(geometry)
DROP FUNCTION utmzone(geometry);
Usage: SELECT ST_Transform(the_geom, utmzone(ST_Centroid(the_geom))) FROM sometable; */
 
CREATE OR REPLACE FUNCTION utmzone(geometry)
RETURNS integer AS
$BODY$
DECLARE
geomgeog geometry;
zone int;
pref int;
 
BEGIN
geomgeog:= ST_Transform($1,4326);
 
IF (ST_Y(geomgeog))>0 THEN
pref:=32600;
ELSE
pref:=32700;
END IF;
 
zone:=floor((ST_X(geomgeog)+180)/6)+1;
 
RETURN zone+pref;
END;
$BODY$ LANGUAGE 'plpgsql' IMMUTABLE
COST 100;