Don't know what type of replacement pump you are using but there have been several cases of multiple faulty replacement waterpumps posted. Leaks via shaft seal and weep hole the common cause. Leaks from mating surfaces between impeller and housing not usually a problem regardless of whether sealant used in addition to gasket or not.
Suggestions of excess coolant pressure (possibly caused by partial blockage by remains of shattered pump) along with faulty pumps have been made, but no definitive answer IIRC.
Both my wps replaced about 25k miles ago in May/02 with no problems to date. Only about 95% of shattered impellers recovered. Replacements were new metal impeller Tredex brand although GMB stamped on shafts.
