There is a picture circulating the Internet that apparently shows a portion of the "campaign code" slip that comes with early copies of Final Fantasy XIII.  From the parts that can be read, it seems to indicate that the code will grant a "secret item" as well as be involved in gaining access to upcoming beta tests.

Besides the fact that parts are cropped and blurry, there is not enough information to say anything is certain at this point.  Final Fantasy XIII will hit stores on Thursday, December 17th, so everything will come to light soon.