Maybe I missed something -- was there something about Elsevier that was particular bad? Worse than Wiley, and Springer? Would it make sense to boycott all the major publishers, or does Elsevier charge particularly more...
I think Elsevier is a good position to act as a kingpin in the math world when it comes to the academic publishing racket. I'm not sure which publisher is worse than others, but I do think it's more likely that success will come against a particular target (more researchers will take the pledge), and that things would change with all publishers as a result. Elsevier has a long list of well-respected math journals, so a victory with them could be most meaningful in the eyes of the mathematicians who choose where to submit their papers.
I've always heard that yes, Elsevier is especially bad. They charge more, and make more profit -- but the higher price comes entirely from the market share lock-in, and not from any additional quality/service.
http://news.ycombinator.com/item?id=3494340