Fox 411 also swears Nathan Fillion will be Ant-Man, but this was denied at Comic-Con by Joss Whedon and Marvel. If fans thought that was mere trickery, Fillion failed to join the assembled Avengers onstage (and he was at the convention all weekend long wearing, among other things, a Green Lantern shirt to boot!). But still, the rumor mill insists the ink is merely drying on his contract.
While I know there are bound to be more surprises coming out of The Avengers production, I think the final line-up is set. From a sheer marketing perspective, it would make little sense to add two more Avengers after Comic-Con. The first official Avengers photo was quite literally seen around the world, and was the first indication non-geek audiences had that there would be an Avengers film. There's no way Marvel would have missed the chance to have Wasp and Ant-Man onstage, especially if they were being played by Longoria-Parker and Fillion. (Miss out on having more pretty faces -- especially another female face -- onscreen? Come on.) Nevertheless, I think there will be stories like this right up until the credits roll on The Avengers, as anonymous sources are going to insist they're in the film somewhere.