In SVN 1.6 a change was introduced, that will allow to have file (before only folders were allowed) listed as part of svn:externals directory property. File externals looks differently in svn status --xml command output and that's why they are not detected.
Here is xml, produced by svn status --xml on a working copy, that have both file and folder externals:
<?xml version="1.0" encoding="UTF-8"?>
<status>
<target path=".">
<entry path="system">
<wc-status props="none" item="external" />
</entry>
<entry path="LICENSE">
<wc-status file-external="true" props="normal" item="normal" revision="16091">
<commit revision="14585">
<author>alex</author>
<date>2011-09-24T12:04:17.516241Z</date>
</commit>
</wc-status>
</entry>
</target>
</status>