This is a new option in the Photos settings, under Cellular data. You can turn on cellular data and sync your iCloud Photo Library over cellular (finally!) but this toggle exists and it says that if you turn it on it "may cause you to exceed your quota". What exactly is this limit? Is it a certain number of photos? Megs? It'll only update a few times per day? My testing iPhone has a 200MB T-Mobile "free data for life" SIM in it, so my ability to test iCloud Photos over cellular is limited. Wondering if those who are brave and updates their main phone have played with this more.