iPads in general are factory unlocked. They are not locked to a carrier.
The only caveat to this, is that Verizon has a registry of all devices for their network, and will not allow other devices onto their network.
It used to be that it was competing technologies at play. CDMA vs GSM , however that has become way less of a concern these days (however, the bands that a device works on can be different internationally vs North America).
I have used two different canadian sim cards, and 2 different us sim cards, as well as a UK sim card and an israeli sim card in one of my ipads. (and my unlocked phones).
Mike