Among cognitive scientists, there are many definitions for the term "mental model." Only one of those definitions is "mental models are the users understanding of how a system works," as Nathan Kendrick states in the thread at IXDA. (I was an a couple of working sessons with Nathan at Wells Fargo a while ago.) The mental model diagram here represents the throughts, actions, philosophies, and emotions of someone going to the movies. I do use the word "task" to indicate the small square boxes in the towers, but only because I need one word instead of four to name it. I'm not terribly picky about definitions of --am much more of a generalist. So "task" is close enough for me, since it represents the "action" part of the concept.

There is another discussion on this site where we debated "mental model" versus "alignment diagram," and I also conducted several surveys over the course of last year. People were pretty much 2 to 1 in favor of calling it a "mental model" despite what the academics might say. Then some academics let me know that the term "menal model" had sprouted several definitions as it has been evolved by cognitive scientists over the past decade. So we decided to stick with "mental model" as the name. "Alignment diagram" would have worked, and so would have something like "experience model." People who have been making these diagrams prefer to stick with "mental model."

So your one-sheet definition for a mental model diagram can say that the top half of it represents a persons actions, thoughts, philosophies, and emotions. The bottom half of it represents the ways your organization serves (or doesn't serve) those concepts. The diagram as a whole acts as a touchstone that your team returns to, say every quarter, to prioritize design projects and double check that decisions are in accord with the strategy your organization follows, which is represented as the proposed (and purposely not proposed) solutions on the bottom half of the diagram. Also nice: a mental model doesn't shift very quickly, so these diagrams hang around for 10 years without much change. This longevity can benefit your organization as team members come and go.

Also for your one-sheets: I develop hypothetical audience segments in order to decide whom to interview for the mental model(s). Then, based on all the interview data, I refine/change these audience segments. Next, I sometimes develop them into personas. This is how "mental model" and "personas" fit together.

You're right, mental models are not built for a project, they are built for the people an organization serves. And you're right again that there are frequently distinct sets of users for the various solutions an organization produces. The very first part of the book outlines the importance of setting up task-based audience segments --or a hypothesis of what you think they are-- which are distinct from one another in terms of their mental models. (See my report on audience segmentation at Adaptive Path for more, or wait for the book to come out. It's possible the PDF version will be ready by the end of August.)

Distinct task-based audience segments will often have different mental models. Each mental model represents the beginning and end of a solution (or solution set) for that particular kind of person. I describe how these various solutions, when they happen to be web properties, can come together in the section of an essay at Adaptive Path called The Intranet Business Park.

So, yes, you would do an exercise--a mental model--for both audience segments. And if their mental models are distinct enough--some overlap, but not a lot--then you would create two distinct solution sets. To give you an example, for enterprise software companies, usually there is a separate web property for potential customers than for current customers. There is overlap in the mental spaces where both segments are looking at improving what they have, but customers need access to bug patches and tech support, whereas potential customers need access to people who can help them decide whether the product is right for them.