As of writing (16:00 JST on Wednesday 18th January 2012) Wikipedia is blacked out apart from one page:
This is in protest at two bills currently being debated in the US Congress (PIPA in the House and SOPA in the Senate). These bills are being rushed through at quite a fast track in congress because they are bi-partisan (meaning: the big businesses who drafted the bills, and are corruptly paying congress-critters in campaign donations for their support, have bought peple in both parties).
In early January there was a movement by some opposed to this bill asking various large Internet organisations to black out in January in a coordinated effort to oppose these bills and raise public awareness about them. Most of the major service providers such as Google can’t really afford a day’s blackout. As Wikipedia is a non-profit and doesn’t make money per eyeball it was one of the few high profile sites to be able and willing to take this step.
There are more details from the EFF about these proposals.