The official Pokémon Twitter feed posted a tweet on Sunday January 6th, 2012 at 7:15am asking the question: "Trivia time! How many possible evolutions of Eevee are there?". With the upcoming announcement on January 8th, 2013, this seems to be hinting to the fact that there will be a new Eevee Evolution revealed that day. A new Eevee evolution would be the 8th one and the announcement is on January 8th.


Currently in Pocket Monsters Best Wishes Season 2, one of Satoshi's rivals in the tournament is Virgil and he has all the current Eevee evolutions: Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Leafeon and Glaceon. Also in Japan, a Pokémon Center Eievui Event just ended there, so Eevee is fresh in everyone's mind. It was also announced that the Movie 16 short that airs in Japanese theaters on July 13th, 2013 is Pikachu and its Eievui Friends (ピカチュウとイーブイフレンズ) which features Eevee and its evolutions.


Source: https://twitter.com/Pokemon/status/287940481814523905

Oooh, that sounds promising. Have to admit, it'd be a strange coincidence if this theory proves to be wrong.