Square-Enix announced at DKS3173 that the Final Fantasy XIII demo will be packaged with the Advent Children: Complete blu-ray release in March next year. Unfortunately for everyone outside of Japan, the demo will probably arrive much later than that.
Nomura has told Dengeki though that the Final Fantasy XIII demo may be sold separately some time after Advent Children: Complete's release. Square-Enix know they can make a huge amount of revenue by including the Final Fantasy XIII demo with AC:C but is selling the demo separately possibly taking it too far? The demo is supposed to be 90 minutes long, so the file size could make it unfeasible to host on the Playstation Network but if they sell it for an extortionate price, would people still buy it? I can pretty much answer my own question and say yes but that doesn't make it right.
Other than this, a few more details about the compilation were discussed which I've listed below:
All of the Final Fantasy XIII titles are based around extremely fast combat.
Final Fantasy Agito XIII will have both command input and action elements in it's gameplay.
Characters in Agito will actually transform into Summons. (misunderstanding) Each character will have their own individual summon.
Agito is currently running on PSP hardware but won't be released until after Final Fantasy XIII and Versus because of the lengthy story and large amount of characters.
SOURCE: Final Fnatasy XIII.net