A few days ago Final Fantasy Union mentioned some details that had surfaced as a rumour regarding the Dissidia: Final Fantasy OST. Well, now Sony had addded it to their website for purchase and the release date remains the same.
The 24th of December 2008 is when it will ship, so if you're importing you probably won't see it this side of Christmas. They've also announced that the Limited Edition will be based on quantity, not time. So if you want to make sure you get your Limited Edition copy, make sure you order now.
More information about pre-ordering the OST on the site.