While you're in the iPhone's Cellular settings pane, I suggest you also change the Roaming setting so that data roaming is turned off. That's particularly important if you have Sprint as your carrier, as its unlimited data plan applies only to data transferred on its network -- you'll pay a fortune for data roaming, even in the United States. In an iPad's Cellular Data settings pane, data roaming is (so far) safe to leave on: Neither AT&T nor Verizon have roaming partners for their iPad data services, so there is no worry about data roaming charges within the states. But do disable international roaming for both data and voice on any mobile device, so you don't accidentally get huge bills when outside the United States. (it's easy to forget to do so once you're on a trip.)
There is one other setting you should double-check in iOS 6: the iTunes & App Store pane in the Preferences app. This pane should be familiar, as it exists in iOS 5 (but called just iTunes). To keep automatic iTunes downloads from occurring over cellular connections, be sure the Use Cellular Data switch is set to Off.
Finally, be careful on an iPhone when using FaceTime that you have a Wi-Fi Internet connection (look for the Wi-Fi icon at the status bar at the top of the iPhone screen), so you don't accidentally have a FaceTime conference over a cellular connection. If you have an iPhone 4S or 5, or an iPad 2 or 3, iOS 6 allows FaceTime conferences over cellular connections, which can quickly eat up your data plan. You can disable FaceTime over cellular on an iPhone via the Cellular settings pane and on an iPad via the Cellular Data settings pane.
Some carriers may disable or restrict FaceTime connections over cellular, to reduce strain on their networks. AT&T, for example, is limiting iPhone FaceTime usage to customers who subscribe to one of its pricey "everything" plans; users with cheaper family plans can't use FaceTime over cellular. On the other side of the spectrum, Verizon places no such conditions on FaceTime. If your iPhone's carrier or plan does allow FaceTime connections over cellular, be careful!
The convenience of connectivity enabled in iOS 6 and other mobile platforms is a great idea, but it typically costs too much to freely take advantage of its promise.
This article, "Watch out! iOS 6 can jack up your phone bill," was originally published at InfoWorld.com. Read more of Galen Gruman's Smart User blog at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.