The message has been "Retweeted" enough for it to be unlocked and it's about as unexciting as we expected. It turns out it's a Chocobo that can be used with your Xbox Live Avatar.
If you're still interested in acquiring the item, you can register on Xbox.com's Final Fantasy XIII page to unlock it for your avatar.
Xbox.com - Final Fantasy XIII Page
Source: FinalFantasy-XIII
If you're still interested in acquiring the item, you can register on Xbox.com's Final Fantasy XIII page to unlock it for your avatar.
Xbox.com - Final Fantasy XIII Page
Source: FinalFantasy-XIII