Quote from their interview with director Joe Johston:
Did you have to think much about the upcoming Avengers movie when working on Captain America?
I really didn’t. Because this was a period film, because this was the origin story, I didn’t have to worry about the Avengers which was a present day story. We have present-day bookends and bring Cap back at the end and then I basically hand him off. And The Avengers is its own thing.
By the way – if you pop Charlie Sheen’s head on top of the Red Skull’s body and adjust his dialogue to be delivered in a German accent, he actually makes a damn fine villain for The Avengers. Vinning!