He said the blackout will affect all users worldwide who attempt to view any English language Wikipedia page during a 24 hour period starting at midnight Washington DC time (5 AM GMT). The exact contents of they page to be used in place of the requested content is still being determined via discussion on Wikipedia.
Whatever the specifics, he said his goal is to "melt phone systems in Washington on Wednesday."
Answering a number of questions about the blackout, Wales made it clear he still considers both SOPA and PIPA to be a real threat. At one point he responded to a claim SOPA had already been defeated with a tweet saying, "Rumors of the death of SOPA may be premature. But Senate version going strong."
January 18 is the same day Reddit will be going dark in protest of SOPA and PIPA. However, Reddit's protest is only scheduled to last 12 hours. Other websites already planning similar protests on that day include include Tucows, BoingBoing, and the entire Cheezburger Network.
A website dedicated to the blackout, set up by Fight The Future, also lists Mozilla, XDA-Developers, WordPress, and many others as confirmed participants.
I wish youtube or google would do this then people would pay attention