I got an email from Sony:
We wanted to make sure you have your voucher code in case you aren't sure where to find it, accidentally deleted your XMB message, or just did not receive it.
Your Voucher Code:
Redeem through Sony Entertainment Network Account Management
1) Visit: https://account.sonyentertainmentnetwork.com/cam/a
2) Enter your voucher code in the space provided and click "continue"
3) Your credit information will appear and click "redeem"
Remember your code expires March 5, 2013.
The Sony Entertainment Network Team
I redeemed the code and it worked fine. There seem to be other people who are recieving it too, so check your inboxes.