A very focused boycott CAN work, for example the current one against Taco Bell to try to force the tomato growers and the buyers to negotiate with the farmworkers. FLOC did it with Mt. Olive (pickles, Ohio & NC) but it took 7 years, I think. Thinking of the corporate campaigns and boycotts I'm familiar with, I'd say the boycott is one useful tool as part of an overall strategy of attack on a particular corporation. It's not a primary plan of attack, but it can inflict some damage. (Detroit News/Free Press, Staley). Without that, boycott is a slow-building long-term knawing effort. What demand are they hoping these sundry corporations will accede to? Or is the idea just to 'withdraw support'?
There's a similar strain in the peace movement here--plant an organic garden, bike to work--it's really about taking a personal-moral stand rather than analyzing a situation and developing a strategy. I think it's useful, when faced with this stuff, to point out just whose lifestyles are being preserved and whose are being devastated. (I love the 'I already sacrificed my healthcare and education...') That's why I'm such a bore about the war not being about lower gas prices at the pump.
(And, well, then there is that idea floating around that our power is concentrated at the point of production and diffuse at the point of consumption...) Cheers, Jenny Brown