SOAPHandler uses SOAPMessageContext to get SOAPMessage.
Then get SOAPBody via SOAPMessage.getSOAPBody().
Since SOAPBody extends org.w3c.dom.Element, use the following to get at <arg0>
Element arg0Element = (Element) body.getElementsByTagName("arg0").item(0);
System.out.println("arg0Element is null: " + (arg0Element == null)); // IT'S NULL

Here's the code:
public boolean handleMessage(SOAPMessageContext msgContext)
{
try

{ // see what's coming in System.out.println("------INCOMING---------"); SOAPMessage message = msgContext.getMessage(); message.writeTo(System.out); System.out.println("------------------------"); // get <arg0> SOAPBody body = message.getSOAPBody(); Element arg0Element = (Element) body.getElementsByTagName("arg0").item(0); System.out.println("arg0Element is null: " + (arg0Element == null)); // IT'S NULL ... }

}

Here's the output:
-----INCOMING--------
<soapenv:Envelope xmlns:jcat='http://catalog.javatunes.com/' xmlns:soapenv='http://schemas.xmlsoap.org/soap/envelope/'><soapenv:Body><jcat:findByKeyword xmlns:jcat='http://catalog.javatunes.com/'><arg0>nn
</arg0>
</jcat:findByKeyword>
</soapenv:Body></soapenv:Envelope>
------------------------
arg0Element is null: true

NOTE: the whitespace nodes in the message are added by JBoss somewhere, the source message contained no whitespace. (Shouldn't matter anyway.)
You can see that <soapenv:Body> indeed has an <arg0> element (in no namespace).
I also tried this with <jcat:findByKeyword> using both getElementsByTagName and getElementsByTagNameNS, both yielding same result.