There has been some controversy over whether the distributed denial of service attacks by Anonymous against Mastercard, Visa, and others are right or wrong.The 2003 documentary Weather Underground is a really good look at the revolutionary group and its times. From the participants' perspective, they had tried everything to stop the war in Vietnam and it hadn't worked. They hoped to spark a revolution through acts of bombing property (note that, with one exception (which failed) and one conjectured exception, they did not attempt to kill people, and in fact, tried very hard and succeeded to avoid injuries). The Red Army Faction in Germany had similar aims, but included violence against people, and the following analysis also applies to them. (To clarify: I don't agree with the RAF's violence against people.)
From the perspective of the members and many other people at the time, the bombings were morally justified. And I'd have a hard time disagreeing. The military and US corporations involved in the war effort and other horrific US foreign policy needed to be stopped or at least hindered, and dramatic bombings seemed like an effective way to do that (at least, more effective than other ways).
The problem was that instead of inspiring revolution, the bombings put people in spectator mode. What will the Weather Underground do next? Instead of what will we do next? The Weather Underground was increasingly cut off from any kind of a mass base. They sputtered out over the next decade. So the use of bombings could be viewed as legitimate and morally right, but strategically wrong.
Compare and Contrast
Two things are different with Anonymous's DDoS attacks. First, they are distributed and participatory, and seeing them happen, people could join them much more easily than trying to join an armed revolutionary group. Second, internet culture has its share of spectation, but much less than the previous, atomized TV-viewing culture.
I think they were useful at showing right away "here is some resistance". Did that push people to go out into the streets? Or did it keep people at home, joining the DDoSing instead of protesting? I don't think there's any way to know the answer to that question. The situation is complicated even in the history of Anonymous, which broke out of purely online actions to coordinated world-wide protests of thousands in the streets against Scientology just three years ago.
Ultimately I am uncomfortable with the unaccountability of these kinds of online protests. If you do a street action, there's a solidity and meaning and legitimacy to physical presence that's hard to duplicate any other way. There's also a democracy to it whereas you don't know if a DDoS is 20,000 people or one person with a 20,000-strong botnet. (Or a government or even corporate agency.)
That being said, the world may owe Anonymous a favour for kicking the whole Wikileaks situation up a couple more notches. Not that there are many notches left.
In the end, it comes back to the question of being a spectator. Whatever DDoS protests others may do, we can't rely on them. The question isn't "what will Anonymous do next?". The question is: what will you do next?