Jury Nulification --our duty?

When a jury finds a defendant not guilty, not because they don't believe the defendant committed the act, but because the jury thought the law, itself, was not valid, it is called Jury Nullification.  The impression I always had was that when a Jury did that it was somewhat "illegal."

However, I received an e-mail today that had these quotes from founding fathers:

Quotations and Comments on Fully Informed Juries

THOMAS JEFFERSON (1789): I consider trial by jury as the only anchor ever yet imagined by man, by which a government can be held to the principles of its constitution.

JOHN ADAMS (1771): It's not only .. (the juror's) right, but his duty ..to find the verdict according to his own best understanding, judgement, and conscience, though in direct opposition to the direction of the court.

JOHN JAY (1794): The jury has a right to judge both the law as well as the fact in controversy.

ALEXANDER HAMILTON (1804): Jurors should acquit even against the judge's instruction...."if exercising their judgement with discretion and honesty they have a clear conviction that the charge of the court is wrong.”