Not quite sure, but I pre-ordered it the day the CE edition was announced for the US. I can't find my slip, which isn't surprising since I throw away my pre order receipts since I have the rewards card anyway and that's all they ask for.
I Made sure to pre-order it early because I'm still ticked off about missing the Borderlands 2 red treasure box edition. My store only had one and someone pre-ordered it on day one. Now I'm the guy who stole the CE :)