Check out the Photography Is Not A Crime (It’s A First Amendment Right) Website. Great news and information concerning all aspects of photographing people (including police officers) in public places.
Excerpt from The Photographer's Right (pdf) by Bert P. Krages II
"The general rule in the United States is that anyone may take photographs of whatever they want when they are in a public place or places where they have permission to take photographs. Absent a specific legal prohibition such as a statute or ordinance, you are legally entitled to take photographs. Examples of places that are traditionally considered public are streets, sidewalks, and public parks.
"Property owners may legally prohibit photography on their premises but have no right to prohibit others from photographing their property from other locations. Whether you need permission from property owners to take photographs while on their premises depends on the circumstances. In most places, you may reasonably assume that taking photographs is allowed and that you do not need explicit permission. However, this is a judgment call and you should request permission when the circumstances suggest that the owner is likely to object. In any case, when a property owner tells you not to take photographs while on the premises, you are legally obligated to honor the request..."
