Is a violent protest ever acceptable?

I've heard a lot said lately about the occupy movement concerning "keeping it peaceful." What I'd like to know is whether it's ever acceptable to have a violent protest? I'm kind of asking if the ends justify the means.

To elaborate slightly, the ancient Romans did a lot to keep the citizens happy and entertained, not so much because of their morals, or pride for their Country, but because if the people got angry enough it was likely they'd storm politicians houses and heads would roll. I know this is terribly simplified but stick with me. Many of the most important political changes in some Countries histories were triggered by riots in some way or another. Some revolutions have started this way

Now I'm not suggesting we go medieval. I'm actually not suggesting anything. I just think it would be interesting to hear if you think it may or may not be justifiable to destroy banks or insurance company buildings, or things of that nature, in a hypothetical situation. If you do think that this may be justifiable, under what circumstances?