Grey Poupon mustard is the only exception. They have been licensed to produce it in the U.S.
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http://www.geocities.com/NapaValley/4079/Glossary/D....

Amazingly, some 90 per cent of all mustard seeds are grown in Canada. Two types of mustard seeds - brassica nigra and brassica juncea - mainly grown right here in Canada - are the only ones used to prepare authentic Dijon mustard! The "Dijon" in mustard is not actually a reference to the city (although growing conditions are ideal - they just don't have as much arable land as we do), but to the exacting manufacturing standards first established there in 1930, and revised in 1937. The only mustard museum in the world is located at Maille's headquarters in Dijon, France.

http://www2.alberta.com/food/columns/displayone.cfm?...