I have no problem with targeted activism and every admiration with the principles behind it, but please actually think about who your target is, here, because "Buy no books/magazines/movies/music whatsoever" is what's getting reblogged on that giant spiffy graphic.
Your local independent bookstore or comic shop, your favorite also-in-fandom and fair-use-activist authors (and recording artists), self-published comic creators -- those people would be hit harder by a blanket boycott on all buying of published media in the month of March than any of the huge publishing companies/RIAA/MPAA.
I chose those 3 specific artists as examples because aside from their popularity in fandom, they all actually have books coming out in March whose sales figures can directly affect whether their next book gets published at all -- or in the case of Terry Moore, comics coming out whose sales go straight to the dude's gas bills, do not pass go, do not pay anybody besides the shipping company he uses to get them out to the comic shops.
So, yeah, I get the idea. I even support the idea; by all means, wait until April to purchase that DVD box set or that new album released by RCA. But be more specific about what not to buy and who not to buy it from, so you don't cut off the nose of your favorite artist to spite SOPA's face.
ETA: from killabeez via morgandawn, here's a great and much more targeted post addressing how to boycott specifically the companies that support SOPA and similar over-reaching "anti-piracy" legislation. There's even an app for that!