Stated by 9to5mac http://9to5mac.com/2012/09/30/verizon-iphone-5-carrier-update-to-eliminate-data-usage-while-connected-to-a-wifi-network/
This evening an update is going out to Verizon iPhone 5 owners which resolves an issue where, under certain circumstances, iPhone 5 may use Verizon cellular data while the phone is connected to a Wi-Fi network. We’re not sure how much data leaks and why or if Verizon will be offering refunds. We’ve reached out to Verizon for a statement.
Instructions to install the update.
1. Tap Settings > General > About.
2. Wait for the following alert to appear:
Tap Settings > General > About.
Wait for the following alert to appear:
3. Tap OK to install the update.
4. Hold the Sleep/Wake button down until you see “slide to power off”.
5. Slide to power off.
6. After the device powers off, hold the Sleep/Wake button to turn your phone back on.
Note: To finish the installation of the update, your iPhone 5 must be turned off and then on again.
After your iPhone restarts, tap Settings > General > About, then scroll down to Carrier and verify that “Verizon 13.1″ is displayed.
The issue may be a carryover from the Wifi+Cellular feature that was in the betas of iOS 6 but was eliminated in the final version. One reader below mentions that the data leak seems to have been occurring while the device was asleep.