Employment is covered under Title 1 of the ADA. As long as your employer is not a church or a federal agency, you have the right to have a Service Dog with you if you are a person with a disability as defined within the ADA. The ADA Amendment Act cleared up a lot of the mess the Supreme Court made in terms of definition.
Sec. 4 is the definition - http://www.eeoc.gov/laws/statutes/adaaa.cfm
employment discrimination on the basis of disability (includes a lot of links) - http://www.eeoc.gov/laws/types/disability.cfm
Title 1 of the ADA - http://www.eeoc.gov/laws/statutes/ada.cfm
Churches who do not rent space to a secular group are exempt from the ADA. Federal Agencies follow the Rehabilitation Act of 1973 - http://www.eeoc.gov/laws/statutes/rehab.cfm