There is a difference between Verison using a photo illustration in an ad to sell their services and me showing a photo in my portfolio as a sample of my work.
In the Verison example, the implication is that the happy person in the ad is enjoying themselves because Verizon is such a wonderful company and offers such great products. The implication is that the happy chap in the photo is a spokesperson.
Now, in the case of my actual, physical portfolio, I'm certain there's no problem showing photos I have taken, with or without releases.
My main concern is an online portfolio. If you were to put someone's image there without their permission, they might be successful suing you for invasion of privacy. The difference is that it's very broadly and widely publicly accessible, not limited to just the relatively select and small number of people I'm able to share my actual, physical portfolio with. As far as I know, this hasn't been litigated yet.
And, at the same time, there are is no release required when posting event photos online for people to peruse and buy prints of themselves or family members. I suppose if someone ever objected to having their photo on the Internet, you might be obliged to remove it. I guess I don't see how this, which I know doesn't require a release, differs from putting your portfolio online.
This is similar, I think, to Dan Heller's point about a photo taken under contract to do portraiture of an individual, that's later used for editorial purposes. The person who hired you to take the photo did not intend it be sold for editorial purposes, so you had better have a release for it, too.
It's quite clear and been proven a number of times that sales of fine art prints is does not require a release here in the U.S. This is considered Freedom of Speech and is Constitutionally protected. Courts would weigh the right of privacy against the right of free speech, and the latter would nearly always win out unless it were a particularly onerous situation.
It's a bit different with a celebrity, whose face and fame are their meal ticket. There you should have a release in all but the most clearly editorial situations, and sometimes even then (say if you were hired to make their portrait).
And, there's a distinction between a fine art print - made in limited edition and only available directly from the artist or at a limited number of outlets - and a large press run of posters widely distributed and retailed. I'm not really clear what "limited edition" means. I've heard the figure 200 thrown around but am not certain this has any relation to needing a release. At any rate, the poster is a commercial venture, so not protected under Freedom of Speech, and would require a release.
Even in some cases of fine art prints, it would be wise to get a release. I'm thinking particularly of nudes, where a person is recognizable (and "recognizable" is not limited to just their face being shown clearly, BTW.)
All these might differ in countries other than the U.S. There may even be some variance from state to state here.
Liable and defamation are entirely separate issues. So, if, for example, you had a "gallery of fat people" online (or anything else that might be considered derogatory), I think you'd better have releases from any and all recognizable individuals.
Overall, the best response so far is simply "always get a release". It's safer/better to do so, than not. The law is in constant flux as new cases go to court and matters are re-interpreted, plus you cannot always predict how you might want to use an image in the future.