There was already an announcement trailer for The Avengers that did the logo-voiceover schtick, so I'd be glad to see Marvel opt for something a bit more… well, live action for their actual teaser trailer. According to Aint It Cool, that's precisely what they have done:
I got word that MARVEL has shot a specific teaser trailer for THE AVENGERS – and that LOKI is featured in that teaser trailer.
Well, that seems fair enough, though I wouldn't be so sure it's actually a teaser trailer yet. Aint It Cool ran a piece on a Tron 3 teaser trailer a while back, and it turned out that the footage in question was shot to provide easter eggs for the Blu-ray release. Similarly, this Avengers footage could be for a "button" scene after the end credits of Thor, or Captain America.
Either way, I think we'll be seeing it soon and it certainly seems we're square on Loki being the big bad for Whedon's storyline (It's how I always heard it was going to go down). But… what about some back-up for that naughty god? Is he the only villain in the frame?
You may consider the following a SPOILER. If so, run and hide. Put a cushion over your face. The rest of us will enjoy this for what it is.
According to Kellvin Chavez at Latino Review, Loki will be seen to get his tricky little mitts on the Cosmic Cube and use it to summon the Skrulls, the ridge-chinned alien ne'er do wells of Marvel lore. Yep, for the one hundredth time, somebody is telling us there will be Skrulls in the movie. Time to start believing it yet?