Remember shortly after Thor returned from the dead, and decided it was time to kick some Ol' Shellhead butt when he discovered Iron man had used a clone of him as a "secret weapon" to kill another superhero in Civil War?
It's kinda like that, only with less emotional baggage and more moving pictures. Check out this morning's clip from The Avengers which sees the two heroes having a not-so-bromantic moment. Squaring off in the woods in what's likely to be an early sequence from the film, the two appear to be holding nothing back, but not really sure of one another's limitations.
Star Chris Hemsworth is on record as saying that Thor has a different idea of what to do with his renegade brother Loki than the rest of the team does, which might be one reason for the disagreement. Also, it serves the old "fight-then-team-up" convention that all comics, but especially Marvel Comics, are notorious for.
After about a minute of some pretty serious scuffling, the clip fades to a montage of images and clips we've seen before, making this clip about half-movie, half-trailer, but completely worth watching.