WFS-T Edit using OpenLayers, Geoserver, PostgreSQL ("error performing insert")

You don't say what versions of the software you are using but if it is a recent PostGIS (1.9+) and an older GeoServer (2.1.x -) then you will need to run the legacy.sql script to add geomFromText() (and other functions) back into PostGIS. See http://postgis.refractions.net/docs/PostGIS_FAQ.html#legacy_faq for more details.