http://download.java.net/jdk6/docs/api/index.html says

writeStartElement(prefix, localName, namespaceURI) writes <prefix:localName xmlns:prefix="namespaceURI"> if namespaceURI is unbound(in both reparing and non-repairing cases). See the html table in the java doc. But the namespace attribute is not written for this call in stax FCS(which is integrated into JAXWS workspace). Attaching the sample program that produces it.

For e.g:

writer.writeStartElement("soapenv", "Envelope", "http://dummy")

produces only <soapenv:Envelope> instead of <soapenv:Envelope xmlns:soapenv="http://dummy">