I have had a lengthy discussion with our appellate department on this issue of jury nullification - and I learned a little along the way. Here's the deal:
If a jury renders a nullification verdict of not guilty, practically speaking, it's game over. Not guilty verdicts cannot be questioned or set aside. As a practical matter, the jurors would not be suject to any type of sanctions because statutes in most states prohibit investigation into the state of mind of the jurors while rendering the verdict.
Is a jury nullification verdict legal? No. There is no statute that prohibits jury nullification verdicts, but the judge always instructs jurors that they are not to consider the law, only the facts. Since a jury nullification verdict is based on a consideration of the law involved, it would be contrary to the judge's instructions and, to that extent, "illegal."
Here is where it gets a little tricky. As mentioned, above, if the fact of jury nullification is not discovered until after the verdict has been rendered, there is virtually nothing that can be done about it. If it is discovered while the jury is still deliberating - a juror sends a note to the judge telling him that there are several jurors who are indicating they are going to acquit regardless of the facts because they disagree with the law - then there is something that can be done.
In this latter case, the judge would call the potential nullification jurors out and question them individually. He would again tell them what their duties are and what those duties are not. He would then ask if they will be able to follow those instructions. If they say no, they cannot in good conscience do that, they would be dismissed, and an alternate seated. Punished? Probably not.
If they say yes, they could follow the judge's instructions, they would be returned to the jury room for further deliberation. If the jury then returned a verdict of not guilty, game over. If the jury remained hung, the judge would repeat the questioning of the jurors, trying to determine the reason for the inability to reach a verdict. If some jurors admitted they were only hanging to nullify, they would be dismissed. If they didn't, deliberations would continue until the trial either reached a verdict or the judge declared a mistrial due to the jury's inability to reach a verdict.
Those who argue for jury nullification have something to argue about. However, claiming that jurors have a "right" to render a nullification verdict is flat wrong. They not only do not have a right to nullification, they are expressly instructed to consider only the facts and not the law, and can at ther very least, be excused from the jury if they refuse to follow those instructions.
That's pretty much all I know about the jury nullification issue. I hope it helps to clear the air a bit.