I'm a bit confused regarding the document architecture. Lets look at MSN for Mac - what would the document be in that application? The contact list? The text we insert to talk to other people?
When the need comes to save or read data into the application, what type of data should it read? Contact lists or chat logs?
Update: Pushing this a bit forward, what is a document? A file type that the application is prepared to open?
Maybe MSN for mac isn't a document-architecture application. Not every program needs to use the same system.
In general, a 'document' is just what you say; the data that makes up your application's files. It can refer to documents on disk - which have a particular filetype, or runtime documents, which are a collection of interrelated model objects in your application.
Your application might support saving/loading documents of various different on-disk filetypes, but they could all be represented with the same model objects at runtime.
MSN clients and other chat applications create and view logfiles. They can't edit them, as far as I know, but that's a feature - they're logfiles after all. There are easier ways to create such logfiles, but there is a nice pseudorandom quality to the way these applications do it which some people prefer. Sort of like SO but different.