
Ant-Man, a founding Avenger who was left out of the Phase One plan will make his introduction into the MCU during Phase Two. Announced during SDCC 2012. As of this time it's speculation if the film will be a late Fall release in November 2014 or another slot. Marvel/Disney may do 3 releases a year during Phase two.
Ant-Man is another Stan Lee/Jack Kirby team-up created character so it's not hard to see why he was chosen. Characters by those guys seem to be adaptable and relatable.
There have been 3 different men to use the Ant-Man persona and early rumors are that the film may feature two of them.
Ant-Man 1 - Hank Pym (original)
Biophysicist and Security Operations Center expert Dr. Henry 'Hank' Pym decided to become a superhero after discovering a chemical substance that would allow the user to alter his size. Armed with a helmet that could control ants, Pym would shrink down to the size of an insect to become the mystery-solving Ant-Man. He soon shared his discovery with his girlfriend, Janet Van Dyne, who became his crime-fighting partner as the Wasp. The duo would become founding members of the Avengers.

Ant-Man 2 - Scott Lang (just back from the dead in comics)
Scott Lang was a thief who first became Ant-Man after stealing Henry Pym's Ant-Man suit to save his daughter Cassie from a heart condition. Reforming from his life of crime, he soon took on a full-time career as Ant-Man with the encouragement of Hank Pym.

Ant-Man 3 - Eric O'Grady (started as a SHIELD agent)
Eric O'Grady is the third character to take up the title of Ant-Man. Eric O'Grady is a low-level agent of S.H.I.E.L.D. who stumbled upon the Ant-Man suit in S.H.I.E.L.D.'s headquarters. A man of few morals and willing to lie, cheat, steal, and manipulate in order to get ahead in life, Eric stole the armor for his own selfish plans, which included using his status as a "super-hero" to seduce women.

I expect Janet Van Dyne aka Wasp to at least appear even if she doesn't become Wasp. Especially with the possibility of 2 Ant-Men for the film.

I intentionally included a load of links and images for people who are not fully aware of the character. Just in discussion of The Avengers film thread a number have let it be known the films are their only portal in thus far. So I wanted to include in this OP a good primer as details begin to emerge, especially a few weeks from now when Edgar Wright really gets started on the project.

My thinking is if they do go with two Ant-Men it'll be Pym and O'Grady. You have to have the original and with SHIELD having such an integral influence in the MCU it's easy to use O'Grady. I'd prefer they don't make him as irredeemable and corny as he was often times portrayed. He could be funny man to Pym's straight man I suppose.
Thoughts on this development and what you might like to see for an Ant-Man movie....it's happening.