I'm not seeing any problem with the wiki at the moment.
You can check the status of the ROS services on http://status.ros.org/ . It looks like they were having some stability problems yesterday.
EDIT Poking a little more, it looks like the core wiki page is fine, but many other pages on the wiki are having problems. This looks a bit like there are some pages in a cache somewhere, and anything that isn't in the cache returns an error.
EDIT Most of the ROS team is asleep right now. I wouldn't expect a resolution on this for another 6-8 hours (around 8pm UTC). I suggest you use the mirrors linked by @IsaccSaito until then.
From China: error 503.
Same problem here (Germany)
Same problem (Poland).
Same problem here in the Netherlands, while status.ros.org is saying everything is fine. http://www.downforeveryoneorjustme.com/wiki.ros.org
Same problem (Germany) (http://wiki.ros.org works, but any link => 503)