Don,
I have a very similar environment (mixed AD & DNN authentication). I captured the request & response details initiated when setting up a blogging account in Live Writer. It looks like the AD http module is capturing the request and redirecting it to the AD logon script, which is subsequently returning 401 unauthorized. I made this request when logged in as a windows user with access to the site, and a blog created. I'm not sure how one would address this just yet, but wanted to give you the details that I found. I will research this some more and post back here if I learn anything more. Ironically, it may be Live Writer that doesn't support a windows-authenticated blog, not metaPost?
Thanks!
POST /metapost.ashx HTTP/1.0
Accept: */*
Accept-Language: en-US, en, *
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Windows Live Writer 1.0)
Content-Type: text/xml
Host: mgweb1
Content-Length: 484
<?xml version="1.0" encoding="utf-8"?>
<methodCall>
<methodName>blogger.getUsersBlogs</methodName>
<params>
<param>
<value>
<string>ffffffabffffffce6dffffff93ffffffac29ffffffc9fffffff826ffffffdeffffffc9ffffffe43c0b763036ffffffa0fffffff3ffffffa963377716</string>
</value>
</param>
<param>
<value>
<string>admin</string>
</value>
</param>
<param>
<value>
<string>dnnadmin</string>
</value>
</param>
</params>
</methodCall>
HTTP/1.1 302 Found
Connection: close
Date: Mon, 25 Feb 2008 23:00:22 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Location: http://mgweb1/DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx?tabid=58
Set-Cookie: authentication.status.0=4A75C2E6F2619E2DD91818825287C9A778ED7BC54F
A053F1C9BC4D48558496E4F8A256A6338076B1BD1F8472B3241E2ABCDE3D4BBBADB695B5C2C4FE
F6DE89D7C710E0E2E68C9D5B54C30D1B1438AAF8BEB7F57DEA56D44C14BBB44526CE95A70D7A38
5A47FFC7BCE485254598FC82A3E2E7105BF2F3B58EF805B65DAC95C617; expires=Tue, 26-Feb-2008 00:00:22 GMT; path=/; HttpOnly
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 212
<html><head><title>Object moved</title></head><body>
<h2>Object moved to <a href="http://mgweb1/DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx?tabid=58">here</a>.</h2>
</body></html>
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GET /DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx?tabid=58 HTTP/1.0
Accept: */*
Accept-Language: en-US, en, *
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Windows Live Writer 1.0)
Content-Type: text/xml
Host: mgweb1
HTTP/1.1 401 Unauthorized
Content-Length: 1656
Content-Type: text/html
Server: Microsoft-IIS/6.0
WWW-Authenticate: Negotiate
WWW-Authenticate: NTLM
X-Powered-By: ASP.NET
Date: Mon, 25 Feb 2008 23:00:22 GMT
Connection: close
Proxy-Support: Session-Based-Authentication
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<HTML><HEAD><TITLE>You are not authorized to view this page</TITLE>
<META HTTP-EQUIV="Content-Type" Content="text/html; charset=Windows-1252">
<STYLE type="text/css">
BODY { font: 8pt/12pt verdana }
H1 { font: 13pt/15pt verdana }
H2 { font: 8pt/12pt verdana }
A:link { color: red }
A:visited { color: maroon }
</STYLE>
</HEAD><BODY><TABLE width=500 border=0 cellspacing=10><TR><TD>
<h1>You are not authorized to view this page</h1>
You do not have permission to view this directory or page using the credentials that you supplied because your Web browser is sending a WWW-Authenticate header field that the Web server is not configured to accept.
<hr>
<p>Please try the following:</p>
<ul>
<li>Contact the Web site administrator if you believe you should be able to view this directory or page.</li>
<li>Click the <a href=" location.reload()">Refresh</a> button to try again with different credentials.</li>
</ul>
<h2>HTTP Error 401.2 - Unauthorized: Access is denied due to server configuration.<br>Internet Information Services (IIS)</h2>
<hr>
<p>Technical Information (for support personnel)</p>
<ul>
<li>Go to <a href="http://go.microsoft.com/fwlink/?linkid=8180">Microsoft Product Support Services</a> and perform a title search for the words <b>HTTP</b> and <b>401</b>.</li>
<li>Open <b>IIS Help</b>, which is accessible in IIS Manager (inetmgr),
and search for topics titled <b>About Security</b>, <b>Authentication</b>, and <b>About Custom Error Messages</b>.</li>
</ul>
</TD></TR></TABLE></BODY></HTML>
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