It was thought at one time that the one exception is a book cover, which was considered commercial in nature because it's considered the part that "sells the book." But it turns out this isn't true either. According to the 11th Circuit court, which ruled on July 18, 2006, that amazon.com did not violate a person's right to privacy or commerce simply because the photo was used as the cover of the book, and that amazon.com displayed it on their website. (The court case and the circuit's opinion is written here. Many have argued that this is similar to posters and postcards, but it's not always so easy to draw such parallels. Rights of publicity are generally created by state statute, but as you can see, the federal courts can intervene, and it depends on exactly what the statute says and how the courts interpret it, before a judgement can be made.
Granted, this is not giving you much solid advice to go on, which itself should be a lesson: there is no solid advice. There are only guidelines. The main point to this section is "art," and in that context, the courts almost always rule in favor of no release, because they like to give art precedence over commerce in the event of a tie. And therein lies your judgment call: is the use more art (or editorial), or more commerce? An advertisement is easy (commerce) and a photo book is easy (art/editorial), but the stuff inbetween is dicier.
The exception to when "art" isn't really art, brings us back to the example where you have a portrait studio in a shopping mall. Remember my point about posting a photo of someone in the store to demonstrate your work? That's a commercial use because you are promoting your business. Here, the photo—or "art" in one of its forms—is being used in a commercial context because it is very specific to the nature of the business. This is quite different than a restaurant, where artwork is not so clearly a part of the establishment's "business model." Here, it's more widely accepted as being part of the decor. (One can argue that decisions to eat at a place aren't based on its artwork.) What's more, it is generally accepted that eating establishments have always been venues for art. This de facto standard has been established long ago, and courts usually uphold such traditions.
Art exhibits—and indeed, the sales of photos as artwork—are exempt from requiring a release from subjects that happen to be portrayed. Courts have decided repeatedly on this matter, including those situations where other potential conflicts may be intertwined. See this New York Post article for a case in point.
However, an exception may apply if the exhibit were displayed in, or underwritten by, an organization in a way that would make the exhibit appear to be more of an advertisement than an actual art exhibit. For example, American Express, the credit card company, once sponsored an exhibit of photographs from Annie Liebovitz, where her portaits of famous people were portrayed. Under each one was a copy of an expired American Express card that was once held by that particular celebrity. Clearly, the exhibt was intended to promote the card, so model releases would be required from each of the celebrities.
A less clear case would be if there were an art exhibit sponsored with an effort to raise money for AIDS research. Here, the legality isn't so much the case as whether it would be in good taste to exhibit photos of people who may not want to be affiliated with the cause, or associated with the disease. Someone who wanted to bring a case would probably have many positions from which to launch. The lesson again is, don't oversimplify. You need to look at the broader context and think objectively.
Overhead presentations are subject to similar constraints. If it's for commercial use, the images might have to be released. But here's a case where pragmatism comes into play: presentations are to screened in front of such a small audience, and usually on a one-time basis, that acquiring a release may be overkill. Of course, this equation changes if the presentation occurs more often, to wider audiences, or is underwritten by a high-profile entity.
