Sulfites are naturally occurring substances as well as food preservatives.
Sulfites are frequently used in the production of wine, as well as in preserved fruits and in some other foods. [link] Some people are allergic or sensitive to sulfites, particularly people with asthma.
The United States government requires the labelling of sulfites on foods and beverages. [link]
For the purposes of paragraph (a)(3) of this section, any sulfiting agent (sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite) that has been added to any food or to any ingredient in any food and that has no technical effect in that food will be considered to be present in an insignificant amount only if no detectable amount of the agent is present in the finished food. A detectable amount of sulfiting agent is 10 parts per million or more of the sulfite in the finished food.
In addition to wine, vermouths, wine-based liqueurs, and fortified wines may contain sulfites.
According to Wikipedia [link]
In Australia and New Zealand, sulfites must be declared in the statement of ingredients when present in packaged foods in concentrations of 10 mg/kg (ppm) or more as an ingredient; or as an ingredient of a compound ingredient; or as a food additive or component of a food additive; or as a processing aid or component of a processing aid. In the United Kingdom sulfites are included in the list of known allergens and should the concentration of sulfites exceed 10 ppm, bottles of wine should bear the warning 'contains sulphites'
The sulfites that can be added to foods in Canada are potassium bisulfite, potassium metabisulfite, sodium bisulfite, sodium dithionite, sodium metabisulfite, sodium sulfite, sulfur dioxide and sulfurous acid. These can also be declared using the common names sulfites, sulfates, sulfiting agents.
"Sulfites: Safe for Most, Dangerous for Some" (good summary of legal history in US) [link to pdf]
"The FDA's Sulfite Ban" Washington Post (1986) [link]