What I haven't read in the replies yet is that service dog laws are set state by state. The Federal Law must be obeyed in all states, but beyond that, states may add to that law.
For instance, in Texas service dogs must be certified to have public access. This means that the dogs must have been trained - or the owner must have worked with - a certified service dog training school.
In my state, anyone can call their dog a service dog. There is no certification process. No letter is needed. Nothing. Also in my state, service dogs in training are allowed anywhere service dogs can go, which is in any area the pubic is allowed in any business except in some areas of hospitals and churches (unless the church allows service animals.) If the service dog "acts up" (ie. barks, disobeys, sniffs merchandise or whatever), the dog may be asked to leave, and the dog must leave.
I do believe because of the proliferation of people falsely claiming their house pet to be a "service dog" to allow it access to public places (airplanes especially) that we will soon be seeing a change in the law that requires some sort of certification for service animals. Shame, shame on those who falsely call their dogs service dogs as this will lead to more red tape for those that truly need their animals.
Another huge problem is the proliferation of owner trained service dogs. These dogs are usually less well trained and lack many important public access skills.
Be sure to check your state's laws on service dogs as each state differs.