According to http://superuser.com/questions, there are currently 0 questions on Super User. http://meta.superuser.com/questions says the same about Meta Super User. The other tabs (featured, unanswered, etc.) are also affected.
The main pages (http://superuser.com and http://meta.superuser.com) display questions most of the time, but those questions are at least 55 minutes old right now; there are newer questions however.
Stack Overflow and Meta Stack Overflow are working fine. All others sites seem to be affected.