We've just learned that the new iPad will have LTE support built-in even when it's sold outside the US and Canada — in other words, there won't be HSPA-only models sold in countries where LTE isn't operational. Those iPads will be capable of provisioning on North American LTE networks if they're brought into a carrier store (presumably, you'll need to obtain a SIM).
Interestingly, Apple tells us that the iPad will either operate on AT&T LTE or Verizon LTE depending on where it's originally purchased — not both. We're reaching out for additional details on how that'll work; stay tuned.
Update: Apple has clarified that international units will function on AT&T LTE.