xsltproc insert unexpected spaces

[danbetz]

Binary package hint: xsltproc

ubuntu version: Ubuntu 8.04.1
xsltproc version: 1.1.22-1ubuntu1

I am using DocBook and want to include some plain text files in my document.
The following code

...
<screen>
line
  <xi:include
    href="../CAST_vs_3DES/3DES.netp"
    parse="text"
    xmlns:xi="http://www.w3.org/2001/XInclude" />
</screen>
...

produces a xml-fo file with the expected result, except of some unexpected
whitespaced in the first line of the included file.

...
e" linefeed-treatment="preserve" font-family="monospace">
line
      === Durchgang 1
  8192 8192 2048 112.00 3.94 &#13;
...

The first line of the included file (beginning with ===) starts in the the first column.
The problem occurs with any verbatim DocBook Tag. I testet some other verbatim tags
like <programlisting> and the <literallayout> tag, but without sucess.