use_doc_date is ignored, file modified date used

Hi,

We have set

use_doc_date: True

but pages are being ranked by file modified date not by any of the 
metadata dates.  We are using these metadata date tags:

<META NAME="DC.Date.Created" SCHEME="ISO8601" CONTENT="2003-06-16">
<META NAME="DC.Date.Issued" SCHEME="ISO8601" CONTENT="2003-06-16">
<META NAME="DC.Date.Modified" SCHEME="ISO8601" CONTENT="2004-08-03">

Why are the metatags being ignored and the file modified date being used 
instead?  This is a problem because the files are part of a content 
management system that serves the pages dynamically, so the file modified 
date is always now (the time of indexing).

Martin