This was the correct answer, and it appears someone already redeemed it.
I don't know if the Xbox website was being wonky, but the code wasn't working at first. Now it says it was used. Weird. o_O
Here's another code for ya'll. Figure it out!
??E7AWNA??
First two question marks: initials for the main character of the Bit.Trip series.
Last two question marks: "Is this real life?"
Here's a code, but I'll make ya work for it! ;)
??NWZR7???
First two question marks: 12, 24, __
Last three question marks: Start with the acronym for the PS3's user interface. Then, flip the middle letter upside-down.