Version details: Bug still exists in 2.6.20

libxml2 doesn't properly handle attributes as being of type ID if the attribute
has a namespace.  In other words,
    <!ATTLIST some-element myns:id ID #IMPLIED>
does not cause any indexing to occur.  The bug is in the xmlIsID() function in
valid.c, which discards the namespace prefix before checking whether the
attribute is of type ID.

Simple test case:

doc.xml:
<?xml version="1.0"?>
<!DOCTYPE my-root [
    <!ATTLIST my-root myns:id ID #IMPLIED>
]>
<my-root xmlns:myns="uri" myns:id="indexme" some-attr="findme"/>

transform.xsl:
<?xml version="1.0"?>
<stylesheet version="1.0" xmlns="http://www.w3.org/1999/XSL/Transform">
    <template match="/">
        <value-of select="id('indexme')/@some-attr"/>
    </template>
</stylesheet>

Running
  xsltproc transform.xsl doc.xml
should print "findme", but it outputs an empty result.