I haven't tried this with child portals, but I do have a dozen or so parent portals using AD Authentication.  Changes are in source from 3.2.2 - I haven't tried this on the newer versions.  If someone has a better fix for this, I would like to hear about it.

Components/Users/UserController.vb

~line 460 - Function AddUser
Public Function AddUser(ByVal objUser As UserInfo, ByVal AddToMembershipProvider As Boolean) As Integer
            Dim UserId As Integer = -1
            Dim Status As UserRegistrationStatus

            Dim _config As Authentication.Configuration = Authentication.Configuration.GetConfig()
            Dim ProviderTypeName As String = _config.ProviderTypeName
            Dim isADSIProvider As Boolean = (ProviderTypeName.IndexOf("Authentication.ADSIProvider") >= 0)

            Try
               .....
                  Else
                        ' the username exists so we should now verify the password
                        Common.Globals.SetApplicationName(objVerifyUser.PortalID)
                        If isADSIProvider Or AspNetSecurity.Membership.ValidateUser(objUser.Membership.Username, objUser.Membership.Password) Then
                            ' the specified password matches the password on file
                            UserId = objVerifyUser.UserID

I am another one with this problem but we are using 4.3.3. We have just launched our Portal with 8 Child portals (Intranet). Users are automatically registered/authenticated (± 30000) on the Parent portal but when they navigate to a child portal it does not work (not logged in or authenticated). If they now browse back to the Parent portal they are logged out and if they try to log in with their AD credentials it fails.

I hope this is fixed soon or someone supplies us with a fix. I will be monitoring this forum for any updates/news.