Dotnetnuke has removed the authentication out of the core and into a more module approach.  This is exciting stuff because now you won't have to reapply your customization everytime you update Dotnetnuke.

Authentication providers (as they are known) are now stored in /DesktopModules/AuthenticationServices

The folder labled DNN is the core authetnication, that you are use to modifiying.

I hope that helps,

Stuart

This is my code to login a user:

   Dim objUserController As New UserController
   Dim objUser As UserInfo = UserController.GetUser(PortalId, ReqUserId, False)
   objUser.Membership.Password = pass
   If objUser Is Nothing Then Exit Sub
   Dim ipAddress As String = ""
   If Not HttpContext.Current.Request.UserHostAddress Is Nothing Then
    ipAddress = HttpContext.Current.Request.UserHostAddress
   End If

   'Try to validate user
   Dim loginStatus As UserLoginStatus
   objUser = UserController.ValidateUser(PortalId, objUser.Membership.Username, objUser.Membership.Password, "", PortalSettings.PortalName, ipAddress, loginStatus)

   If objUser Is Nothing Then
    Select Case loginStatus
     Case UserLoginStatus.LOGIN_USERNOTAPPROVED
     Case UserLoginStatus.LOGIN_USERLOCKEDOUT
     Case UserLoginStatus.LOGIN_FAILURE
    End Select
   Else    'Login Success
    Dim updatePassword As Boolean = False
    Dim updateProfile As Boolean = False
    'Set the Page Culture(Language) based on the Users Preferred Locale
    If (Not objUser.Profile Is Nothing) AndAlso (Not objUser.Profile.PreferredLocale Is Nothing) Then
     Localization.SetLanguage(objUser.Profile.PreferredLocale)
    Else
     Localization.SetLanguage(PortalSettings.DefaultLanguage)
    End If
    'Complete Login
    UserController.UserLogin(PortalId, objUser, PortalSettings.PortalName, ipAddress, False)
   End If
   Me.Response.Redirect(DotNetNuke.Common.NavigateURL, True)

Note this code uses a UserID instead of name. You'll want to change it to use a username.

Peter