It actually is according to their FAQ
Can I watch movies instantly on more than one PC or Netflix-ready device?
Some membership plans allow you to watch simultaneously on more than one personal computer or Netflix-ready device. If you are on the Unlimited Streaming plan, the Unlimited Streaming + 1 DVD out-at-a-time plan or a limited streaming plan, you may watch only one device at a time.
If you are on the Unlimited Streaming + 2 DVDs out-at-a-time plan, you may watch on up to two devices at a time. Members on the Unlimited Streaming + 3 DVDs out-at-a-time plan may watch on up to three devices at a time. The maximum is four devices at a time -- available for members on the Unlimited Streaming + 4-or greater DVDs out-at-a-time plan.
Although, I just have unlimited streaming and it's not limiting me to a device number yet. I think I've had 3 going at once.