It would go where every other page goes. The MVC is nothing but a way to split the backend code from the code that displays a page, from the code that controls what backend code gets used.
So your login page would be nothing but an added action for the controller, a new backend code (or model) part, and then either a new or reused view.
For a better idea, think of it this way:
Controller is index.php
Everything that is done has an associated action passed via a $_GET parameter to index.php like so: index.php?action=xxxx
index.php sees this action and includes the appropriate files and calls the appropriate functions.
So, for your login example, you'd have a index.php?action=login which displays a form that posts to index.php?action=login2. Now the controller reads the action URL parameter and includes the correct file, login.php, and then calls the proper function (login or login2). The functions deal with the $POSTed information and set $SESSION or cookie information as needed. Then the controller includes the correct template file and the template file displays the results or whatever.
For a good idea of what I'm talking about, go ahead and look at Simple Machines Forum as they have a good setup for it.
