xml2po writes placeholder tags to the POT files,
e.g. it sometimes fails to handle phrases with quotes:

Applying xml2po to the following XML file...

    <?xml version="1.0" encoding="UTF-8" standalone="no"?>
    <!DOCTYPE formalpara PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                         "http://www.docbook.org/xml/4.3/docbookx.dtd">
    <formalpara>
     <title>
      <phrase>Title phrase with <quote>quoted</quote> text</phrase>
     </title>
     <para>Blah blah</para>
    </formalpara>

... produces this output:

    #: foo.xml:6(quote)
    msgid "quoted"
    msgstr ""

    #: foo.xml:6(phrase)
    msgid "Title phrase with <placeholder-1/> text"
    msgstr ""

When I translate the above string and merge the source XML file
with a proper po file (using xml2po's "--po-file" option), in the
resulting XML file this placeholder tag will eventually be replaced
with "<quote>my translation</quote>".

However, a translator should never be bothered with the internal details
of the transforming program, he should never see any "placeholder" tags.
(Just imagine 10 placeholders...)

There is a workaround which *seems* to fix this issue:
    a) adding "quote" to the list of ignored tags, and/or
    b) adding "phrase" to the list of ultimate tags.

But this will not really fix the bug, since it does not change the
algorithm.  It is still possible to get placeholder tags in the POT files,
depending on the list of ignored and ultimate tags.

You might want to check (and test!) the following patch, which tries to
fix the algorithm.

Regards,
Ulf