Title I of the ADA applies to the workplace. It's not exactly the same as Title III which applies to public accommodations in that access is not necessarily automatic. You may have to provide your employer with proof of your disability and need for a service animal in the form of a letter from your treating physician. Other state and federal laws may also apply in some instances.
Issues involving employment and service animals should be directed to the Equal Employment Opportunity Commission.
To find the EEOC office near you, check the EEOC web site at http://www.eeoc.gov/offices.html.
You also can call the EEOC at 1-800-669-4000/1-800-669-6820 (TTY).