In Java, one defines the namespaces for the XPath evaluator using the class NamespaceContext. In Java 6, the XPath evaluator does not consult the defined namespace context to interpret the default namespace. This despite explicit discussion of the default namespace in the Javadoc.
The workaround is to avoid using the default namespace in the XPath query.
It would appear that this somewhat surprising behaviour is consistent with the XPath 1.0 specification. Section 2.3 (Node Tests) states:
A QName in the node test is
expanded into an expanded-name using the namespace
declarations from the expression context. This is the same way
expansion is done for element type names in start and end-tags except
that the default namespace declared with xmlns is not
used: if the QName does not have
a prefix, then the namespace URI is null (this is the same way
attribute names are expanded). It is an error if the QName has a prefix for which there is
no namespace declaration in the expression context.
(emphasis is mine)
