If you go to see Captain America The First Avenger remember to stick around for the trailer AFTER the final credits - if you're a Marvel fan you won't regret it!
After the credits there is a teaser trailer for The Avengers, which is going on release in 2012 (4 May 2012 in the US).
I haven't been able to find an officially sanctioned version of the teaser yet, so here's one which was obviously recorded by someone in a cinema (and a noisy one at that!):
The Avengers (official teaser trailer after... by mediafreaks3d