Add me to the list. Verizon iPhone 5. Use WiFi most of the time, and my data usage appears surprisingly high.
Ran a test: Connected to WiFi, reset data usage statistics, did a speedtest, ran a very short youtube video, and Cellular Network data went from 0 to 66KB sent, 801KB received.
Checked wifi router speed from my laptop, and there are no connection or speed issues (10Mbps down, 1Mbps up, 67ms ping), so there should be no good reason for LTE to kick in.
Sounds like I'll be making a visit to the genius bar in Orlando.
Edit: Disabled iCloud backup, although it was only set to backup on WiFi. I do keep my contacts on iCloud, so that and some other stuff is turned on. Documents and Photostream are only set to sync when on WiFi.