"Some membership plans allow you to watch simultaneously on more than one personal computer or Netflix ready device at the same time. If you are on the 1 disc out at-a-time plan [or stream-only plan], you may watch only one device at a time. If you are on the 2 discs out at-a-time plan, you may watch on up to two devices at the same time. Members on the 3 disc plan can watch on up to three devices. The maximum is four devices simultaneously, and that is available for members on the 4 or greater discs out at-a-time plans."
It looks like this text has always been in effect, and that Netflix is just now getting around to enforcing it.