If you are trying to enter in a gamestop digital pre order code and are getting an invalid code error, remember that gamestop did not support pre loading for their codes, and the codes go live at 12:01am PST. So they won't work until 2:00am CST, 3:00am EST, etc.
For those (like me) eager and impatient! :-)
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Still not working. Gamestop was blaming Playstation network. They stated to me that the code should work 'midday.'