You know how some crazy birthers always say that Obama can't be a "natural-born citizen" because he was born with dual citizenship? (Actually, he was born with the potential of being a dual citizen, but chose not to become one.)
In any event, while doing some research (another long story) I came across an Act of the French Congress passed during the French Revolution on 26 August 1792 which conferred French citizenship to a number of worthy foreigners, including Jeremy Bentham, Thomas Payne, Alexander Hamilton and... George Washington and James Madison!
More under the squiggle...
There seems to be some evidence judging from the Digest of American Law: Notes and comments (1824) that Jefferson might also have been granted French citizenship, but I can't find any French sources to confirm it (so far).
This means that two -- possibly three -- of our presidents were French citizens during their mandates: Washington (1789-97), Jefferson (?) (1801-09) and certainly Madison (1809-17), who was elected as a dual citizen.