My views, see Lawyer for more accurate information and I would consider my views only valid in US and maybe other former UK colonies.
Rule #1 always get a release if you can.
Rule #2 if you can't get a release then you need to determine is it news, art or commercial
- If news then you don't need a release (it is good to have one, but not necessary)
It is news if there is an event associated with the photograph or you are using the photograph to depict a situation or condition or make a statement and not trying to sell a product directly from that photograph. While a newspaper may appear to be using the photograph to sell the paper, they are really using the story or statement that the photograph is of to sell the paper (a subtle distinction perhaps, but important)
- If Art you probably don't need release, but it is better to have one. It is highly situation dependant. It is art if you are selling the picture itself separate from any other event or product. Although the photograph may be of an event or a product, you are only selling your expression of that event or product not the event or product itself.
- If for commercial use then must get release.
It is commercial use if you are making money from a product being sold through the use of the image. A calendar is a product. The photo itself is not the product, it is a means to sell the product.
Portfolio images are a wierd exception in that you are ususually using them to advertise the photographer's skills, which would make it commercial. However, as long as you are not selling the photos themselves from the portfolio then they are generally covered by fair use as you are only depicting your skill, it is still a good idea to get a release when possible.
This is the definitions I use to help me determine if I need a release or not.
On my personal site, except for the "event" pages, I will only place images of a person on the site if I have a signed release from the person. The event page I consider to be covered by freedom of the press and don't worry about releases I probably would not be able to aquire anyway.
If you know I am wrong on any of the above please feel free to correct me and state why.