AJ.    Until there is an answer to my post above you should use this script in your login.ascx file.  Change the "domain".  It is not as flexible as Tom's but it works for me.

<script runat="server">  
Sub change(sender As Object, e As EventArgs)
if txtUsername.Text="admin" then
 txtUsername.Text="admin"
elseif txtUsername.Text="host" then
 txtUsername.Text="host"
else txtUsername.Text="domain\" & txtUsername.Text
end if
end sub 
</script>

Ok I did further testing and used the first example posted by Chis Wood from this thread and it works just fine.
http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/89/threadid/60815/scope/posts/Default.aspx 

Then I used Tom's changes and it does not work.
txtUsername.Text = String.Format("{0}@somedomain.com", txtUsername.Text)

There must be something different about our domain setups where I cannot use the email ID for logons.

Hello,

I'm trying to use this script on DNN 7 and it seems that the login.ascx  file is different, I modified the code as following:

<script runat="server">
Sub change(ByVal sender As Object, ByVal e As EventArgs)
 Select Case txtUsername.Text
 Case "admin"
 Case "host"
 Case Else
    If txtUsername.Text.StartsWith("somedomain\") Then
        'The user added the wrong somedomain information
        If txtUsername.Text.Contains("somedomain\") Then
            txtUsername.Text = txtUsername.Text.Replace("somedomain\", "")
        End If
        txtUsername.Text = String.Format("somedomain\{0}", txtUsername.Text)
    Else
        'The user didn't enter the somedomain so add it for them
        if txtUsername.Text.EndsWith("@somedomain.com") then
        else
            txtUsername.Text = String.Format("somedomain\{0}", txtUsername.Text)
        End if
    End If
 End Select
End Sub
</script>

<div class="dnnForm dnnLoginService dnnClear">
    <div class="dnnFormItem">
        <asp:label id="plUsername" AssociatedControlID="txtUsername" ontextchanged="change" runat="server" resourcekey="Username" CssClass="dnnFormLabel" />
        <asp:textbox id="txtUsername" runat="server" />
    </div>
    <div class="dnnFormItem">
        <asp:label id="plPassword" AssociatedControlID="txtPassword" runat="server" resourcekey="Password" CssClass="dnnFormLabel" />
        <asp:textbox id="txtPassword" textmode="Password" runat="server" />
    </div>
    <div class="dnnFormItem" id="divCaptcha1" runat="server" visible="false">
        <asp:label id="plCaptcha" AssociatedControlID="ctlCaptcha" runat="server" resourcekey="Captcha" CssClass="dnnFormLabel" />
    </div>
    <div class="dnnFormItem" id="divCaptcha2" runat="server" visible="false">
        <dnn:captchacontrol id="ctlCaptcha" captchawidth="130" captchaheight="40" runat="server" errorstyle-cssclass="dnnFormMessage dnnFormError" />
    </div>
    <p><asp:LinkButton id="cmdLogin" resourcekey="cmdLogin" cssclass="dnnPrimaryAction" text="Login" runat="server" /></p>
</div>

I don't get any errors, but it simply does not do nothing, if I use the username minux the domain prefix I just just get the wrong username/password error, If I add the somedomain\ prefix it works.

 Is there something that needs to be done differently on this version?

Thanks in advance for any help.