Jumpng off track a little (this post was largely written before The_Eevee_General posted), I got a sudden past-midnight urge to think loudly about abilities.
We know that in every generation since the third, new abilities have been released. And, most new abilities were given not only to the Pokémon introduced in the same generation, but also from older generations. The few exceptions that I know of are signature abilities (such as Flower Gift or Air Lock). Every time new abilities have been released, they have been distributed across generations.
This logically means that the new abilities we see in Gen. VI should also be available to Pokémon from Gen. I - V, somehow.
Thing is, how can they introduce them? Most Pokémon already have two abilities to be available during in-game play, and one hidden ability reserved for "special encounters" such as Dream World, events or the Hidden Hollows. Could they possibly slap a fourth ability to some Pokémon's roster? Would the fourth ability be exclusive to even-more-special events, or will there be two hidden abilities? And wouldn't the poor single-ability 'mons start feeling slightly left behind now?
We know that many Gen. VI Pokémon can have two possible abilities. Inkay, Malamar, Dedenne and Bunnelby (probably among others) all have two confirmed abilities. This points in the direction of the "two normal, one special" ability setup we've had in Gen. V. Still, I don't think it would make any sense to "fill in the holes", that is, only give new abilities to Pokémon without three different abilities already.
Could the Dream World abilities be a thing of Gen. V, being replaced by new "special" abilities in Gen. VI? Like some Gen. III move tutors stayed relevant well into the fifth generation (are any of them still?), will the only way to get some Dream World abilities in the future be transferring a Pokémon all the way from Gen. V, or by obscure events?
Perhaps the new abilities will be available for old Pokémon, but only when they Mega Evolve? Will GameFreak throw a curve ball, and let Gen. VI abilities stay exclusive to Gen. VI Pokémon? There are many new abilities that fit thematically well with old Pokémon (Fur Coat Mamoswine, Refridgerate Regice, Strong Jaw Feraligatr). Like old Pokémon tend to get new moves, they also tend to get new abilities. It's the way of keeping the power creep even between generations, everybody get their share of the new tools. I think we'll see new abilities on old Pokémon somehow, and I'm intrigued to see how.
P.S.: And what about the seventh generation? I wonder if the entire Ability system will survive this decade.