So I've been convinced for a while that Sylveon was flying, but changed my mind given the following evidence:
Of all the originally special types: grass, fire, water, electric, psychic, ice, dragon and dark - only dragon has not been represented with an eeveelution. Similarly, no eeveelutions have received a traditionally physical type!
Sylveon's colouring is red and blue - the traditional dragon colours in the ruby/sapphire and related type sprites.
"soft" dragons such as dragonair and altaria have been released - it's perfectly possible for a pokemon to be dragon without being vicious. Similarly, let's use the reverse argument: all eeveelutions are cute, so if they *were* to make a dragon type eeveelution, what would it look like? Answer: Sylveon.
Dragons are mythical, sylph is related to that. OK that's more of a stretch than the other arguments but again it's as close as a dragon eeveelution could be. What else are they supposed to call it?
Honestly I'd be very surprised if Sylveon turns out to be a type other than dragon, it makes too much sense for it to be dragon!