Verizon Updates the iPhone 5 to Address Cellular Data Use on Wi-Fi

Apple:

This carrier settings update resolves an issue in which, under certain circumstances, iPhone 5 may use Verizon cellular data while the phone is connected to a Wi-Fi network.

Since this is a carrier-specific update, I'm guessing Verizon is at fault here. They should forgive first-month overages for all iPhone 5 early adopters.

Via 9to5 Mac