Can't embed on here. Here's the link. A bootlegged copy of the trailer. Kinda hard to see but looks damn good! http://gizmodo.com/5822377/the-avengers-trailer-leaked-and-its-rad
Well it's likely that the only way to see the trailer is to stay after the Captain America movie. I haven't seen the trailer posted officially anywhere...yet. So the next best thing is somebody with a camera inside the theater recording the trailer and posting it online.
I would imagine if you're interested in the Avengers movie then most likely you'll be seeing Captain America anyhow. You'd be better served sitting through the credits and watching a high quality version of the trailer then cheating yourself and watching some hand-filmed version of it.
I saw Captain America yesterday on opening day. It wasn't bad. I knew there was going to be something at the end of the credits but I didn't stick around. I know it'll end up on the web sooner or later. Regardless, there's a ton of hype around it. I feel The Avengers is going to smash some serious records on opening day in May next year.