I recently returned my AT&T ipad3 and got a Verizon and can confirm the following:
1. It is possible to initiate AT&T service online, not from the iPad, all that's needed is the ICCID (the sim card number printed on the sim) and the IMEI from an ipad1 or ipad2. If you use the IMEI from an ipad3 (a Verizon one) AT&T will not allow you to initiate service.
2. You cannot maintain your AT&T account from the settings section of the Verizon iPad, you must go to
www.att.com/ipadlanding to make changes to the account.
3. There is no way to turn off the auto-renew feature from either the settings section or the online method. The only option you have online is to change the amount of data or cancel the plan. The cancel option online is not the same as the cancel auto-renew feature that is in the settings section. If you select cancel from online, your account is IMMEDIATELY cancelled and your data, as well as your account, are deleted. I know, because I did this. I thought I was canceling the auto-renew.
I had to call AT&T this morning and explain what happened. Surprisingly, I was only on hold for a few seconds, the person had me reestablish an account by putting in my ICCID and IMEI (the phantom one) and selecting the cheapest plan. Once I had done this, she reversed the charge for the $15 plan and gave me back the 3 GB plan that I had originally had. I had used about 1/2 a gig, but the plan had a full 3 gigs.
So, until and unless AT&T somehow prevents the use of an IMEI other than the one actually accessing the data, the Verizon iPad is the most versatile choice because you get the benefits of the more widespread Verizon LTE and the faster fallback AT&T 4G. And I can't see any reason why AT&T would limit this ability since they're not losing anything, like they are when someone uses an unlimited sim from their iPhone.