The speculation came up from an ad found in the latest Shounen Jump (a weekly Japanese manga publication) with a caption on the bottom telling fans the first purchases of FFXII might contain campaign codes for FFXIV.
News of this is getting a mixed reception from Japanese bloggers. Some welcome any chance they may have of becoming a beta tester and others like GIGAZINE (a large Japanese webzine) are worried of how this will work out if it is true. Another likely possiblity is that these codes will only yield in game prizes similar to a campaign that ran for Final Fantasy XI.
Check out a few pictures of the ad that has been creating all the buzz.
Full Final Fantasy XII Ad in Shounen Jump
Final Fantasy XIV Campaign Code with Purchases of Final Fantasy XIII