One of the new features on iOS 7 is the ability to toggle cellular/mobile data access on a per app basis. Settings > Cellular or Settings > Mobile and scroll down to the bottom. To cater for the many people out there currently without an unlimited data plan, I've asked Skype many times to add in a "use on the Wi-Fi only" option to no avail. It appears that iOS 7 may have solved this problem. Having changed the toggle to off for cellular/mobile data for Skype, it warned me the next time I opened the application that this was the case. Great stuff as now people can leave Skype on without worrying about it affecting their data allowance. Will be useful for many other apps too, I expect.