So far what I've been hearing is that the Photo Stream and music backups should be used only on WiFi networks, and if you are careful about doing that a normal 2GB/month data plan will suffice nearly anyone for their data usage. iCloud is pervasive throughout the OS and the internet capabilities of the device are obviously an integral part of making that work. From what I know, there is no reason to be worried about having to pay over-cap rates for data once installing the new OS and making use of iCloud.