PokeBeach had a leaker who successfully predicted the existence of Parabolic Charge, and he gave WPM the English names of the Pokemon 24 hours before anyone else had them. At this point, there was enough evidence to assume him confirmed, and he said that there is a new Fairy type.
Since then, the leaker was forced to sign a NDA, and his friend, the Beta who leaked to him first, was fired.
So, yeah. Your call on if it's true.
Talking about users: ToS Violation Talking about Smash, inFAMOUS, feminism, and hacking: A-OK
The first was from someone who helped leak the gen 5 Pokemon, and the other was from someone who leaked the names of Pancham, Helioptile, etc. before they were officially released. So there's reason to believe both sources.
First there was speculation starting of with Sylveon and its name in all languages including nymphs, fairies and sylvans. As the eevee line is the epitome of typing it is quite strange as no existing type fits sylveons name pattern (or look for that matter)
fairy or magic in general seem also quite fitting in the typechart as nothing already present seems to fit magic while stuff like air-type or beast-type can be represented by already existing types (and no: Psychic type is not magic. Psychic is telekinesis, reading minds and mindcontrol while magic is... creating something from nothing, divine interaction and whatsoever)
Then of course the may or may not be confirmed rumor on pokebeach which talked about the introduction of the fairy-type.