UPDATE: Joss was apparently just joking around. Nathan Fillion will NOT be Ant Man [io9]:
“No that was a joke. Unfortunately there is no such thing as sarcasm at Comic Con. That’s why I explained that he’s not actually Ant-Man – although by the way I think he would be great – Ant-Man does not appear in The Avengers. I just like to play with Nathan. And if you ever get a chance to ask him to do his Ant-Man audition, you will not be disappointed.”
Original article follows below
Joss Whedon has just announced that Nathan Fillion (“Castle”, “Firefly”) will play Ant Man in The Avengers. Whedon made the announcement during his “The Joss Whedon Experience” panel.
Fillion would also, presumably, play the character in the main feature Ant Man which is still in development from director Edgar Wright.
The Avengers is scheduled to hit theaters on May 4, 2012.
Source: Bleeding Cool