You need to change the user registration security settings.  From the admin menu, choose site settings and then advanced settings.  In there change the user registration to public.   It sounds like you have it set to private. 

By way of follow-up, I have been working on a solution since I originally posted my message, and here is what I came up with (for version 4.09):

In admin/Users/User.ascx.vb:
Change
Public Sub CreateUser()
to
Public Funtion CreateUser() As Boolean

At the end of the CreateUser function, add the following line:
Return createStatus = UserCreateStatus.Success

In admin/Users/ManageUsers.ascx.vb, cmdRegister_Click Sub:
Change
ctlUser.CreateUser()
to
If ctlUser.CreateUser() Then
  Dim setting As Object = UserModuleBase.GetSetting(PortalId, "Redirect_AfterRegistration")
  If Not CType(setting, Integer) = Null.NullInteger Then
    Response.Redirect(NavigateURL(CType(setting, Integer)))
  End If
End If