According to the Americans with Disabilities Act, service dogs are allowed in all areas where the public is normally allowed to go. State and local governments, businesses, and non profit organizations that serve the public must allow service animals.
Unfortunately, business owners - particularly when it comes to establishments that serve food - seem unwilling to adapt to the law, and the news fill up with instances of people denied service. The most recent that passed my desk is a story of Big I's diner in Massachusetts, who denied service to a war vet and his service dog. The owner would not back down and allow the dog even after the police arrived to explain the law.
The ADA defines a service animal as any guide dog, signal dog, or other animal individually trained to provide assistance to an individual with a disability. An animal that meets this definition is considered a service animal whether they're licensed/certified or not. Documentation is not required.
A business owner is allowed to ask if the dog is required because of a disability, and what task the dog is trained to perform. They are not allowed to insist on proof of state certification. A service animal must be permitted to accompany the person to all areas where customers are normally allowed.
Click here to read more about service animals and businesses! The Department of Justice also has an ADA information line at 800-514-0301 (voice) or 800-514-0383 (TDD).