After some earlier confusion, Apple has clarified that buyers of the new iPad outside the US will be able to provision the device on AT&T LTE when they're in the States:

iPads sold internationally will be able to connect to AT&Ts LTE network when in the US with the appropriate SIM and plan.

There'd initially been some question as to which 3G-capable SKU — the AT&T LTE or Verizon LTE model — would be sold in European countries, but it now appears that Verizon's model is a carrier exclusive, at least for the moment. Meanwhile, Europeans traveling to the US will be able to provision their iPads on AT&T's 4G network if they've got access to an appropriate AT&T SIM.