Generating page numbers in xrefs to programlisitngs

When I give an xreflabel to a programlisting like in the following

<programlisting id="prg.Conway" xreflabel="Conways Game of 
Life"><textobject><textdata 
fileref="Code/ConwaysGameOfLife.C"/></textobject></programlisting>

and later I reference this by

"...like we have seen in <xref linkend="prg.Conway"/>..."

then, while transforming to pdf, XEP says

{?unresolved internal destination: 'prg.Conway'}

and in the pdf output I see

"...like we have seen in Conways Game of Life [?]..."

Is ist not possible to generate page numbers to cross references other 
than component and sectioning elements?

Greetings,
Joachim

-- 
Joachim Ziegler                       Stuhlsatzenhausweg 85
Max-Planck-Institut für Informatik    66123 Saarbrücken, Germany
Email:           Tel.: (+49) 0681 9325-127