It's true that if you use unlockit.co.nz to use your SIM card from an iPhone. This however violates your user agreement and AT&T reserve the right to cancel you at any time. I got caught once with the original iPad. I'm still grandfathering unlimited data.
But, as mentioned above (you apparently didn't read the entire thread), it's highly likely you can get a new and unregistered SIM for iPad from AT&T (they should give you one for free, just tell them you bought a used 3G iPad without a SIM card) and use it to sign up for a PAID data plan (pre-paid or post-paid, your choice) with AT&T. That would be ENTIRELY legal and AT&T would be glad to have your business.
This is fantastic news!! Just by easily changing the APN, the Verizon LTE iPad can be used on EITHER network! You'll only get LTE on the Verizon network but if you are ever stuck in an area with just Verizon CDMA coverage, for the cost of however much data you need from AT&T, you can switch your Verizon LTE iPad over to AT&T's HSPA+ network (if available in that area, of course) which beats the pants off of CDMA. (CDMA = 1mbps HSPA+ = 4-8mbps).
It's two, two, two networks in one iPad and if done via a new and unregistered AT&T SIM there is zero possibility of complaint from either network.
jsnuff1, THANK YOU for testing it out and for the video!!
Mark