Today, Verizon updated its official My Verizon Mobile application on iOS, adding a Notification Center widget, showing data usage under the Today tab. While there are some App Store apps that can already do this, none of them are as accurate as one getting data figures straight from the carrier.
The widget can be added after updating the app, simply by swiping down the Notification Center, tapping the "Edit" button, the tapping the "+" sign next to the Verizon widget. My Verizon Mobile can be downloaded for free in the App Store, let's hope other carriers do something similar.