Nathan Fillion has already responded to the rumors that he might play Ant-Man by announcing that he has no time in his schedule even if there were an offer, but Whedon says that there isn't. "Although by the way I think he would be great," said Whedon. "Ant-Man does not appear in The Avengers." Oh well."
Now: maybe this is just wishful thinking but I think that just because Ant-Man isn't in the film it doesn't mean that Hank Pym won't be in it. Hank Pym has taken on the identities of Giant Man, Goliath, Yellow-Jacket, and even Wasp. What if this is just Joss' way of throwing us off a little bit? Personally: I'd love to see him as Yellow-Jacket. It would fit into the military look a lot more than any of his other characters, in my opinion. The black and gold suit would look great alongside the Wasp's black and gold suit. Now: this is MOST LIKELY just wishful thinking and I highly doubt that he's in it at all, but let's be optimistic until all bets are off. Shall we? I know that I'm going to try to be. Just sayin'.