Note: WotC has revealed all cards in the Innistrad block. There are 264 cards, a rather large set.

Said to be upcoming in Innistrad Block (Released on Sept.30th, 2011) 9/30/11
Double Sided Cards.
Double Sided Planeswalker: Garruk the Relentless/Garruk the Veil-Cursed with 5 abilities.
New Liliana Vess
Chance of getting 3 rares in one booster pack. Every pack of Innistrad comes with at least 1 double sided card. That double sided card may be rare also, in addition to the original rare in the pack, plus if there is a foil, it may also be rare. Summing up to a chance of 3 rares in 1 pack. WINNING!
Darkness is a common theme.
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Dark Ascension (2nd Set in Innistrad)
So far, only two things: one of which is a fact, the other, is a rumor.
1.Fact: A New Sorin Markov is coming out with Dark Ascension, and he is also depicted on Innistrad packs/publicity.
2.Dark Ascension is said to take place in Ravnica (City of Guilds)