iDownloadBlog's Jeff Benjamin has taken a look at the new "Wi-Fi + Cellular" option that will be in iOS 6. On the surface, the concept is brilliant. If you encounter issues with your Wi-Fi connection, iOS 6 automatically will tap your data plan as a back-up until you're back on the Wi-Fi successfully.
This scenario can occur you've made a Wi-Fi connection and it turns out to be pretty bad. The connection to the router is solid, but the Internet itself is pretty horrid -- case in point, most hotel rooms I've stayed in the past couple of years. Benjamin cites cases where you're making the transition between a place with Wi-Fi, such as your home or a coffee shop, and places without. There's a middle ground where you're barely hanging onto a Wi-Fi connection.
There are restrictions to this, for the frugally minded. You can't be downloading an app that's bigger than 50 MB, be using a third-party browser or an app that streams audio or video content. Benjamin managed to get FaceTime working using Wi-Fi + Cellular, but receiving a call forced the app to hang.
Most people will not notice any sort of change to data consumption, but you can toggle the option off if you're worried about hitting a data cap. While we don't know if Apple will have this option on by default, it won't hurt to make sure once iOS 6 is live.