Wikipedia doesn't seem to be sending 503 responses (or some such). They are simply overlaying the blackout page via javascript. So disable javascript in your browser and you have good old wikipedia as usual. Also, their mobile site isn't blacked out.
Respect for Wikipedia on not sending 503<p><a href="http://blog.nparashuram.com/2012/01/wikipedia-unblackout.html" rel="nofollow">http://blog.nparashuram.com/2012/01/wikipedia-unblackout.htm...</a>