That would be good to know, I have bought a couple of games through Amazon without problem but I might've been lucky.. or I did some research back then, I have selective memory :P
XCOM and Civ V seem to be the 2 games that I was considering grabbing from Amazon US but found out they were region locked. Mafia II also was reported as region locked, but can't recall others off the top of my head.
I traded XCOM that I got in that deal earlier this year, it was not region locked to the US at the time. The person I traded with said they found Amazon sells worldwide region-free keys for XCOM. I only got it at the time to get Civ V Gold, as I had XCOM and Civ IV (drm free from Amazon) already. Funny, it was $15 (after a $5 dollar creadit) and cheaper than simply upgrading my Civ V to the gold edition on Steam. Amazon has some amazing deals sometimes.
Edit: With conflicting info, I'd hate someone ends up buying and finding otherwise. I just know it worked for who I traded with.