Model releases seem to be the source of more confusion than almost any other aspect of photography for aspiring (and established) professionals. I am asked about them during every class or presentation that I give. The irony is that model releases are remarkably simple.
Keep in mind that I am not a lawyer (nor do I play one on TV) and what I am about to tell you should not be interpreted as legal advice of any kind.
Much of the speculation and talk on line about releases is usually off the mark. Whether or not you need a model release has nothing to do with where you are standing when you take your photographs. In fact, releases are not required in any way for taking the initial photograph.
While you cannot normally take photographs in certain military installations or in many private spaces without permission, as a rule you can pretty much photograph anywhere. For example, though many policemen in NYC will tell you that you cannot photograph in the subway, in fact you legally can do just that. Similarly, just because someone tells you something about model releases does not make it true.
The question of whether you need a model release revolves solely around how the image is used, not made. If the image is used in an editorial context such as a book, newspaper, magazine or fine art exhibit, you do not need a release. Any use that is under the umbrella of freedom of speech does not require a release. This is true even if you make money off the image. This does not mean that people in authority over the various outlets where image are actually used will not ask for releases.
On the other hand, if the usage of the image has any kind of implied or stated endorsement then a release is required. Though advertising is the obvious example of such an endorsement, there are many other image uses that can require releases. Book covers, as a rule require releases, because they are used to sell something, in this case the actual book. Ironically, the images inside the book will not generally require a release.
These are the broad outlines of the rules about model releases. There are many other details to know when thinking about model releases. Here are a few good web resources that explain model releases in greater detail:
Understanding model releases is important and I encourage you to look these over. When you need them (or when you do not) is determined by how the image is used and by a series of legal precedents, not based on what someone says on the web (including me!)