So you will understand my background and how it allows me to speak with some authority on this matter, I feel it important that the reader know that in addition to being a working pro for 40 years and having published thousands of photographs in that span of time, I also serve as the executive director of ASMP, American Society of Media Photographers for fifteen years. ASMP is the pre-eminent trade association of publication photographers is the USA. Part of my work was to make sure ASMP members had information like that which follow. Although I left ASMP to semi-retire in 2003, the basis of the following information has not changes since then.
The following information is based upon laws in the USA. It applies in all 50 States. In Canada, United Kingdom, and Australia the laws are similar, but this information should not generally be applied in those Countries without further research. In other countries the laws are different from more strict to almost non-existent. Like all legal matters, you never want to be the test case. It pays to do some additional homework about the matter rather than make assumptions. If you shoot professionally it pays to hire an attorney who is an expert on the subject, and most attorneys are not. In lieu of an attorney a pro ought to join ASMP, the trade association with an authoritative industry voice.
There are two issues in your question not one. They are the right to photograph and the right to us the photograph.
The right to photograph --- In a public place, like on the street or in a city or national park, you have the right to shoot anything you see without permission from anyone unless a permit is required to shoot as in the case of having a lot of gear (like lights and tripods0 on location. Permits usually apply only to commercial shoots, and I have never seen a case in which a amateur or pro was required to get a permit if only carrying cameras and lenses. On private property such as a performance center, sports stadium, or building the management of the property can restrict your right to take a photo. That restriction does not have to be posted, and they can just come up to you and tell you to stop photographing.
The right to use a photograph --- It makes no difference where you shot a photograph, even if it was wrongly taken in a private space when it comes to the right to use the photograph. The First Amendment of the Constitution guarantees your right (free speech) to publish or display your photograph once you have taken it. HOWEVER, the Constitution contains an implied right to privacy also so people have a right to protect their privacy and invading it is a civil offense. FORTUNATELY, the courts and legislators have found a way to ease the Constitutional tension between free speech and the right of privacy. That is done by defining when publication or display WILL violate the right of privacy.
A model release is required when a photograph is going to be used for ‘trade or business purposes,’ which are advertising and promotional uses of the photograph. In general, anytime any business is identified with the photograph, like an ad, brochure, business’ website page selling or promoting the company, etc. a model release is required. When the photograph is to be used for display or publication with no commercial message associated with it, no release is required. Such publication or display is an exercise of free speech. So having an online gallery qualifies as free speech even if you sell prints from it. But use the same photo in a brochure advertising your online gallery and you need a release.
A simple example is this. Look at a newspaper or magazine and you will see photos that are informational as part of a publication’s content and photographs that are promotional as part of the publication’s ads. If you had the same photograph in both places in a magazine, you would need a release for the promotional use and no release for the informational use.
If anyone reading this want to know more about when you need permission to use a photo and how you give permission for someone else to use your photography, let me recommend a book I co-authored called licensing photography. It will provide you with more than you will probably ever need to know. You can find it here:
http://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Dstripbooks&field-keywords=licensing+photography&x=10&y=18
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Weisgrau
http://www.gallery.me.com/weisgrau 
, so are there any official or unofficial rules/ethics about this in the photography community? I guess parents are very sensible to this regarding their own children.
