Title: XML Schema does not allow for validating xml:id in policy documents

Description: The XML Schema document for the WS-Policy LC WD is not yet updated. The reason is that the latest revision http://dev.w3.org/cvsweb/~checkout~/2006/ws/policy/ws-policy.xsd?rev=1.6
has a problem with the schema update for the resolution for
http://www.w3.org/Bugs/Public/show_bug.cgi?id=3560 (enable the usage of
xml:id):
If you use xml:id at the <Policy> element in an instance with the
current schema, you will get an error. You can't use xml:id, typed as an
ID, on an element which has an ID attribute (like wsu:Id) specifically
declared for it.

Justification: Validation of a policy document against the policy schema should be possible also if xml:id is used. 

Target: XML Schema document, with a reference to its latest and dated version from the XML Schema namespace document at http://www.w3.org/2006/07/ws-policy

Proposal: There are three solutions for the problem:
  a) publish two flavors of the schema, one with xml:id, one with wsu:Id
  b) declare two subtypes which are accessible via xsi:type:
<Policy ... xsi:type="xid-flavor" xml:id="boo" ...> versus <Policy ...
  xsi:type="wsu-flavor" wsu:id="boo" ...>
  c) delete the explicit reference to <xs:attribute ref="wsu:Id"/> .
This would make both xml:id and wsu:Id extensibility points.  
I have a high preference for c), including a note in the WD to warn
schema users (don't use other ID attributes than these two), to avoid
the confusion created by deleting the explicit reference to
<xs:attribute ref="wsu:Id"/>.
I have not opened an issue on this and not reopened 3560, since the
normative text has precedence over the schema anyway.

See the discussion thread at http://lists.w3.org/Archives/Public/public-ws-policy/2006Dec/0029.html