Wikipedia are probably doing this so they don’t lose their search ranking for the day but it’s lazy. They should have put in a temporary redirect for every URL. Still, I can’t complain because it does make it pathetically easy to bypass.
You have a few options.
NoScript or Ad Block Plus
For Adblock, go into the filter rules, and add a custom filter to block
http://meta.wikimedia.org/w/index.php?title=Special:BannerLoader&banner=blackout*. This should target just the script that is hiding things and leave the rest of the site working as expected.
A line of code
As a web developer, my natural instinct was to see how they did it and if jQuery was available. After two minutes, I had things fixed. You can reverse the entire thing by pasting the following into your address-bar:
I suggest you save that as a bookmark. You can right click the following link Unblock Wikipedia and save it to your bookmarks. In Firefox, I suggest you add it to the Bookmarks Toolbar. So now when you’re on Wikipedia, just click that bookmark and the site springs back into action.
Edit: Ninja reflexes
Per the comment from Danny Hage, you can just click the stop button at the right point (after the page has loaded but before the SOPA banner loads). Timing is key here. If your internet is too fast, you might struggle.
It’s a bit fiddly with a mouse so you can just hit Escape instead.