Picked up a Verizon iPad Air today and put in an unactivated SIM into it. It would would say "No Service" on the top left and I was not able to connect to activate the account. However once I connected to wifi and internet connection, I was then able to go through the steps to activate the account. After I set up the account, entered all the information and paid....only then did the iPad connected to ATT's towers...LTE and 3G.
What's interesting is that with the Verizon SIM inserted I don't have to connect to wifi....it picks up verizon signal right off the bat. The same was true, from what I remember, with my ATT iPad2.
SO in summary....Verizon air is not locked and will work on ATT LTE....but at least in my case, I had to have an internet connection in the first place to set up an account with the unactivated ATT SIM.