I checked my Kindle Paperwhite 2 a few minutes ago and haven't seen any kind of update yet for Kindle Unlimited yet. I also checked my iPod Touch and there's no update yet of Kindle for iOS.
I don't doubt that we'll see them soon though, I'm looking forward to seeing how Kindle Unlimited is integrated into the Kindle Paperwhite interface and also the Kindle for iOS app.
Edit: They have updated
the Kindle page at Amazon to include information about Kindle Unlimited so clearly they are aware of it as a selling point for Kindles. I hope they hurry up with updates to the KP and Kindle iOS app.