I don't think Ant-Man will be featured prominently in the Avengers movie. They've said that the Avengers movie will feature the Hulk, Captain America, Iron Man and Thor -- basically, those superheroes who have had (or will have had by that point) their own movies. Marvel's working on an Ant-Man movie, but as far as I know we don't have a release date, so it might not be until after the first Avengers movie. My theory is that they'll be in production for Ant-Man at the same time as for the Avengers, or at least not far off, and that they'll have an Ant-Man cameo in the Avengers as a teaser for the Ant-Man movie. Basically, use the Hulk, Iron Man, etc. etc. as advertisement for the Ant-Man, who's a lesser-known member in the non-comicbook-reading world.

yeah i like the idea for an 'after-credits cameo'(forget what it's called), and nick fury says "this will be your new recruit"

They're really getting into the movies as advertisements themselves -- Nick Fury in Iron Man as advertisement for Iron Man 2; Iron Man, the Hulk as advertisement for the Avengers, buzz about the Avengers as advertisement for Captain America and Thor, in turn advertisement back at the Avengers, and all of it eventually funneling back to Ant-Man (which no one knows about) and all of it possibly heading toward an Avengers 2. Though that wouldn't be until 2012 at the earliest, and probably not until 2013, so maybe I'm getting a little ahead of myself here.