It's for Android devices, for now.
Netflix today announced "Smart Downloads," which automates the process of deleting videos you've watched — and then automatically downloading the next one in the series.
So if you've downloaded, say, the first three episodes in a series and then watch the first ep, it'll automatically delete that file and start downloading episode 4. As you'd expect, this only works when you're connected to Wifi. It's also an optional feature — you can turn it off in the app settings, if you want.
We're not seeing Smart Downloads live in our most recent update of Netflix for Android, so stay tuned. You can get the Netflix Android app here.