It is seemingly impossible to get a straight answer from Apple on this question. I actually called AppleCare this morning and the guy directed me to a representative from AT&T. I know that LTE coverage is locked to the carrier you choose at purchase time, but as far as I know, the iPad 2’s 3G coverage was not carrier locked.
I have no reason to trust this site called iPad 3 Buy, since it seems pretty spammy, but it at least gives me hope:
The answer is the third generation tablet from Apple will be unlocked but still is locked with some networks for 4G LTE.
If you have 4G LTE facility in your country, you need to buy the iPad 3 which works on your carrier. But, if your country doesn’t support 4G, you don’t need to worry about this since all the New iPad’s will work universally on 3G.
So who knows. I haven’t found anything in Apple’s own material that describes the rules around this (perhaps because it is complex), but I am hopeful I’ll be able to use the French SIM I just purchased in the iPad as well as my unlocked iPhone 4.