In the UK, it depends on whether the purchase was subsidised by the carrier. For example, we can purchase iPad's from Orange for a discount, but they are LOCKED to Orange SIMS (I have one of these, so can confirm from experience), whereas any iPad purchased at full price from Apple or a retail outlet is UNLOCKED (my Wife has one, and again, I can use any SIM in it).
I'm not sure if the same is now true in the US (I believe everything was unlocked at launch of iPad1), but the best indication is whether or not the iPad2 cost you full retail price (in which case it will be unlocked) or whether it was significantly discounted by AT&T, in which case it will likely be locked.
Hope that helps.