I would tend to assume that Marvel would use the Ultimate Avengers as a basis for any movie they did as it wouldn't have such a long history and cast of characters to deal with. And they would have the advantage of a more "modern" storyline to base the movie on. Anyway I posted this cast in another thead:
Captain America
Thomas Jane : Punisher
Iron Man
Oded Fehr : The Mummy
Nick Fury
Avery Brooks (if they can't get Sam Jackson) : American History X, Star Trek: DS9
Thor
Sean Bean (also possible for Cap. America): Lord of the Rings
Giant Man
Paul Bettany : A Knight's Tale, A Beautiful Mind
Bruce Banner
Adrien Brody : The Villiage, King Kong, Thin Red Line
The Wasp
Kelly Hu : X2, the Scorpion King
Black Widow
Kate Beckinsale : Van Helsing, Underworld
or
Jolene Blalock : Enterpirse
Herr Kleiser
Kenneth Branagh : played nazi Reinhard Heydrich in Conspiracy, Hamlet