By default, cellular downloading on an iPhone is enabled for four services: iCloud Documents, iTunes, Passbook updates (for iOS 6's new e-ticket service), and Reading List (a type of bookmark in Safari that stores not only the URL but the HTML pages themselves for later reading). On an iPad, the four services are iCloud Documents, iTunes, FaceTime, and Reading List; the iPad doesn't support Passbook.
You can burn a lot of cellular data if you leave the iCloud Documents and iTunes switches set to On. If you use Reading List extensively, it too could consume more cellular data than you expect. If you set these switches to Off, updates occur only via Wi-Fi. On an iPhone, I recommend you leave the Passbook Updates switch to On, as you likely will want updated tickets at the airline check-in counter or movie theater, and they shouldn't change often.
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.
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