I'm guessing that Sylveon will be a gender specific evolution of a female Eevee, maybe with extra help from a held item or something like that to combat continuity issues. I also think it may be a normal-type. I wasn't really completely sold on the gender-specific idea at first, until I saw today's episode.
Today's episode had Virgil and his collection of eeveelutions, including a FEMALE Eevee. They didn't have to specify the Eevee's gender, but they did.
I personally think Virgil is gonna show up again after Episode N and that Eevee will evolve, or will already have evolved, into a Sylveon. They have six months to kill between Episode N and when X and Y come out, so I can see them show-casing a few X and Y Pokémon. (Dear god what are they going to do for six months, they've practically done everything)
This is just my guess anyway, based on the fact that it just seems obvious to me that they have that Eevee in Virgil's team for a reason. I could be terribly wrong though, but guessing is fun!!
Also, Ash Pokédex'd every single eeveelution in that episode.