Are online denial of service attacks by WikiLeaks' supporters the modern-day equivalent of the physical, non-violent protests of civil rights and labor rights activists in years past?
Operation Avenge Assange is an attack against infrastructure, not people. Nobody is hurt in a DDOS. Therefore, it is physically nonviolent.
Operation Payback has a particular set of demands, particularly, radical free speech on the Internet. Their target, the financial organizations that froze WikiLeaks' assets, had compromised financial neutrality and worked to block the political activities of a client. As Charles Arthur, the Guardian's technology editor pointed out, you can still donate to the KKK through PayPal. So why was WikiLeaks targeted? Because they chose the United States government as a political target.
That said, I don't think that their attacks are necessarily illegal or immoral. As long as they don't break into other people's computers, launching DDoS should not be treated as a crime by default; we have to think about the particular circumstances in which such attacks are launched and their targets. I like to think of DDoS as equivalents of sit-ins: both aim at briefly disrupting a service or an institution in order to make a point. As long as we don't criminalize all sit-ins, I don't think we should aim at criminalizing all DDoS.
Most interesting of all, Babbage over at The Economist dives into the world of the protesters, describes their methods and finds:
Anons do understand their limitations. The ones I talked to know that to take down a Swedish prosecutor's website does not halt the prosecution in Sweden. They described their motivations, variously, as trying "to raise awareness", "to show the prosecutor that we have the ability to act" and "damage and attention". This is all that a denial-of-service attack can do: register protest. It is not cyberwar. It is a propaganda coup. And it's limited to a limited set of websites: vulnerable, but important.