Matt Damon as Captain America
Matt Damon is probably the only A Lister I can see as Cap. He has the right physique, looks enough like Steve Rogers and can bring in a large audience. He seems like a choice Marvel would go with.
Robert Downey Jr. as Iron Man
Who else?
Chris Hemsworth as Thor
I've only seen him in Star Trek, but I thought he was pretty good for having such a small part. I'm looking forward to seeing how he will do as Thor.
Jeremy Davies as Hank Pym
Better know as Faraday on Lost. He has the right mixture of nice guy and sleaze bag to play Pym
Gemma Arterton as The Wasp
She's a good actress and provides some nice eye candy.
Jeremy Renner as Hawkeye
I love this rumor. He was great in The Hurt Locker and I can definitely see him as Clint.
Scarlett Johansson as Black Widow
Everything we've seen of her looks great so far. Can't wait for Iron Man II.
Samuel L. Jackson as Nick Fury
Duh.