Update: 12/24/2015 8:56 pm.
This outage was caused by an attack from some external entity, which hit our servers with too many requests to handle between 3p.m. and 5 p.m. One server was back up and running by around 3:30 and therefore many websites were up and running. The defensive measures we have taken to thwart the on-going attack may have impact to certain services within WordPress. We will be evaluating the next steps as we learn more.
There was a WordPress outage today 2/24/15 beginning at roughly 3 p.m. and lasting approximately a half hour. The cause of the outage was a flood of requests and we are investigating the potential source as well as how to prevent this sort of problem in the future. I will update this blog when we know more.