Ok, I figured out the problem for my instance (using ADSI provider).  There IS a bug in the ADSI provider in conjunction with the UserManager in DNN.  Here's what the problem is:

In the UserManager, the UserDataGrid tries to bind to "LastName".  However, since the ADSI provider doesn't populate this field in the Users table when adding "Windows/AD" users, it ends up being the literal "Nothing" in the UserInfo object when binding the UserInfo ArrayList to the UserDataGrid in the UserManager screen (see: "admin\Users\Users.ascx" and "admin\Users\Users.ascx.vb" for complete details).

There is a "not-so-elegant" way to overcome this problem without having to re-compile:

Open the "admin\Users\Users.ascx", and find the line:

 <dnn:textcolumn datafield="LastName" headertext="LastName"/>

and change it to:

<dnn:textcolumn datafield="FirstName" headertext="FirstName"/>

This prevents the ascx page from trying to BIND to the "LastName" member of the UserInfo object.

I think this should only have to be a temporary work around.  I'll add a bug to the http://support.dotnetnuke.com site, and all people who are experiencing this problem, please post a response with your feedback to the CASE #.  I'll come back here and post the case # when I get it created.  This will help ensure the CORE team sees that it is a natural problem for us who use DNN in an intranet environment using ADSI.

Thanks!

-- Will Gillen
-- Swirlhost Inc.
-- www.swirlhost.com