As a wanker, I think the progressives who endorse the Black Bloc vandalism and even serious violence as justified because it is overmatched by much worse state violence arrayed against it are missing the power of the legitimacy of non-violence as both an end in itself and a means to an end.
My pompous opinion is that if the Black Bloc wants to use violence against property to raise the issue of property not having the rights as humans they should do this independently of the larger overwhelmingly non-violent progressive movements. Specifically, don't use the WTO rallies as a platform for this kind of direct anti-corporate actions that most of us have a problem with. It does us wankers more harm than good as a movement. Unless, that is the Black Bloc are a giant COINTELPRO style disaccreditation program by the FBI - which I, in my self-important delusion, doubt. The Earth Liberation Front understands this principle perfectly well. Why not Black Bloc?
This is the same issue that destroyed the social progressive movements' cohesion in the 1960's (not that I have any real knowledge of this era): whether and when violence is justified.
Violence is wrong whether it is anarchist violence or state violence. Two wrongs make more wrongs not a liberation of mankind from violence. We keep coming back to this same quandary whether the revolution/repression advocated is from the left or from the right. Violence begets violence begets violence begets violence. You don't get a better world in the long run by making it a worse world in the short run. It just won't work and we wankers have a long history to show us exactly how and why.