I think there is a slight difference.
From answers.com,
reactant: A substance participating in a chemical reaction, especially a directly reacting substance present at the initiation of the reaction.
reagent: A substance used in a chemical reaction to detect, measure, examine, or produce other substances.
For example, hydrochloric acid is the chemical reagent, that would cause the reactant calcium carbonate to release carbon dioxide. Naming as a reactant or reagent is a matter of convention or perspective.