I have read that the Model is responsible for maintaining state among HTTP requests, for example in this article:
The Model is responsible for maintaining state between HTTP requests.
Basically any data whether on a database, in a file, stored in the Session, or cached inside APC must be preserved between requests and thus forms part of the state of the application at the time the last request was completed. So always remember that the Model is not just the database. Even the data you get from web services can be expressed as a Model! Yes, even Atom feeds! Frameworks which rattle off introductions to the Model, almost never explain this upfront which only exacerbates misunderstandings.
When we use some implicit Session objects (say in PHP, JSP etc.,) does it mean that this Session is a Model object?