It works fine. I am using an ATT SIM in a VZW iPad without problem. I am getting full HSPA+ (8-10MBps) download. No LTE, but their "faux" 4G works fine.
Edit: The only issue is you can't set up the iPad data account on the VZW iPad. You need an already active iPad sim or you can probably set it up online at www.att.com/ipad and click on "Add more prepaid data."