It seems by my testing that mikeh is right and it is not possible to have autologin work in mixed mode.  I did however modifiy the httpmodule.authentication authenticationmodule.vb to test for local address prior to redirect

This is from 3.3.5 in the OnAuthenticateRequest sub.

                If (AuthenticationController.GetStatus(_portalSettings.PortalId) = AuthenticationStatus.Undefined) Then  'OrElse (blnWinLogon) Then
                    AuthenticationController.SetStatus(_portalSettings.PortalId, AuthenticationStatus.WinProcess)
                    Dim url As String
                    'mod check if local address*****************************************
                    'this allows for mixed mode with auto login
                    If Not InStr(Left(Request.ServerVariables("remote_addr"), 3), "11.") = 0 Then
                        If Request.ApplicationPath = "/" Then
                            url = "/Admin/Security/WindowsSignin.aspx?tabid=" & _portalSettings.ActiveTab.TabID.ToString
                        Else
                            url = Request.ApplicationPath & "/Admin/Security/WindowsSignin.aspx?tabid=" & _portalSettings.ActiveTab.TabID.ToString
                        End If
                        Response.Redirect(url)
                    ElseIf (Not AuthenticationController.GetStatus(_portalSettings.PortalId) = AuthenticationStatus.WinLogoff) AndAlso blnWinLogoff Then
                        Dim objAuthentication As New AuthenticationController
                        objAuthentication.AuthenticationLogoff()
                    ElseIf (AuthenticationController.GetStatus(_portalSettings.PortalId) = AuthenticationStatus.WinLogoff) AndAlso blnWinLogon Then ' has been logoff before
                        AuthenticationController.SetStatus(_portalSettings.PortalId, AuthenticationStatus.Undefined)
                        Response.Redirect(Request.RawUrl)
                    End If
                    'old dnn code
                    'If Request.ApplicationPath = "/" Then
                    '    url = "/Admin/Security/WindowsSignin.aspx?tabid=" & _portalSettings.ActiveTab.TabID.ToString
                    'Else
                    '    url = Request.ApplicationPath & "/Admin/Security/WindowsSignin.aspx?tabid=" & _portalSettings.ActiveTab.TabID.ToString
                    'End If
                    'Response.Redirect(url)

                    '***********************************************

So let me get this straight. If I do the following:

1) Enable Windows authentication in Admin > Authentication
2) Make the above edit to authenticationmodule.vb.

Then I will allow my intranet-users to automatically logon, if they add the site to the Trusted- or the Intranetzone. And I will still have the Logon-form? Or will I have the windows-popup-logon-box?

Thanks in advance