The Nintendo site, Pokemon site, as well as accredited fansites Serebii, Pokebeach, and Filb have not announced December 31 as the release date. Is it safe to say that Amazon does in fact have the date or are they just using a placeholder? I am not going to order from here, but I would like the know the date all the same.
FireRed/LeafGreen dropped in the fall here, so it isn't always spring. Generally, you want to add about six months, plus or minus two, to the Japanese release date for Pokekmon games: Japanese games that dropped in the fall usually come out in spring here, and vice versa. Since the new game is coming out in June in Japan, we should expect to see it sometime between November and February, most likely December for a Christmas release.
For what it's worth, Bulbapedia currently says "Autumn 2012" for North American release. Not that that means anything.
Also, keep in mind that Nintendo often inflates far-off release dates and revises them to be sooner later. I had this trouble when Black/White dropped; I walked into the store a month before release to pre-order and was told it came out the following week!
Nintendo of America, Nintendo of Europe and Nintendo of Australia/New Zealand all have said the games are going to be released in Fall/Autumn 2012 (or Spring 2012 for Australia/New Zealand because they are in the Southern Hemisphere so their seasons are reversed). If you do not believe me go to the Pokemon website, then video games and it will say Fall 2012.