The Wiki shut down is in response to upcoming SOPA legilation, which can/will have a deliterious effect on 1/2 of the InterWeb.
Yeppers. Reddit is also participating.
It saddens me to know that people haven't heard of this. The "blackout" has been in the works for some time now.
Unfortunately more highly popular sites like Google and Facebook were supposed to join in on the SOPA protest, but backed/pussied out (money......).
Edit - Surfing around, I just came across another site that's shut down in protest. Good stuff.