Jury Nullification Is a Moral Obligation
When summoned for jury duty, I was prepared to serve on a jury for up to two weeks, at which point I had a non-refundable plane ticket so I could get to one of my regular consulting gigs in another state. As it so happens, my jury duty was limited to waiting at the courthouse to be called to a courtroom and being cut loose at about 2pm when it was clear that no more jury selection was going to take place that day. With California's recently implemented One-Day or One-Trial [pdf] system of jury duty, this is the norm. To wit, most people are able to discharge their obligation for jury duty by calling in to the court for five days or showing up at the courthouse for one day.
In those instances where a potential juror is assigned to a courtroom and/or chosen as a juror, they'll probably be done with their jury duty in five to seven days. In those rare instances where jury duty is expected to last longer than that, the court will probably exclude those people who would suffer a financial hardship. In sum, jury duty under the One-Day or One-Trial system is much less onerous and has dramatically reduced the motivation for potential jurors to seek to be excused.
If you are motivated to get excused from jury duty notwithstanding the fact that you are already at the courthouse and/or in a courtroom, you can probably get yourself excused from serving on a criminal trial by revealing any bias that you have against the prosecution (or the defense); in a civil trial, you can probably get yourself excused by mentioning the concept of jury nullification. Of course, if you really believe in the concept of jury nullification, the time to bring it up/act on it is during jury deliberations.
Far from an excuse, jury nullification is a moral obligation. To wit, it is an effective weapon to be used against The Man when he tries to put someone in jail under the pretext of an unjust law, and it works just as well when the black letter law [pdf] unjustly favors a particular civil litigant. As such, if you truly believe that the law as it is currently being applied by the courts is unjust, then you have a moral obligation to keep your thoughts to yourself until after you have been selected for a particular jury. Then and only then should you make your case to your fellow jurors that the law is unjust, and that it should be ignored.
Even if you are unable to convince your fellow jurors that the law is unjust and/or that it should be ignored, your single vote of "not guilty" will cause a mistrial in a criminal case. As such, if the prosecution wants to put that particular person in jail, they will be forced to prosecute their case against that person all over again. This doesn't necessarily mean that justice will be served in the long run, but for one bright shining moment, your voice will be heard loud and clear.
Some might say that jury nullification is a moral hazard, and that the law should be observed, even when it is unjust. However, the true moral hazard in this situation arises from the fact that too many people who serve on juries defer to the authority of the court or the anonymous authority of the crowd rather than listening to the small, still voice of their own conscience.