The teaser shown at Comic-con months ago that featured a scene where Loki appears to cut off Thor's arm was leaked, along with almost 30 minutes of mixed footage from the movie.

For those who can't wait a few more weeks, a transcript of a promotional clip in Beijing was translated (from Chinese) depicting various scenes from the movie.

This is not 30 minutes straight footage, but cut scenes from different parts, similar to a super-sized trailer. There might be some spoilers, but it's highly unlikely for one to complete the plot from this.

The final translation comes from Comic Book Movie:

"It begins with Loki wearing handcuffs in Asgard. Guards escort Loki to Odin's throne where Frigga tells Loki not to repeat his past mistakes. Loki retorts what is "repeat?" To punish Loki, Odin informs him that he will be permanently locked away in a dungeon, and tells him that his brother Thor will be the king. 

Then the scene cuts to Thor and the Warrior Three fighting in a village. Thor and Sif have a banter. Sif is seen shielding herself from an arrow. The scene concludes with Thor smashing a stone-like creature (Kronan). 

Next up, a brief dialogue scene between Thor and Heimdall, as they look at the stars from the end of the Bifrost (Rainbow Bridge). 

Cut to London: Jane is on a date, with a man. Darcy comes interrupts the date by telling Jane that her machine is active once more. A nearby television surprises them with a report that Dr. Selvig has been arrested for streaking at Stonehenge. 

Cut to dungeon: Loki shouts to his mom, "He's not my father!" Frigga asks him "So, I am not your mother, either?" Loki tries to be calm down, and his emotionless reply, "You are not." Frigga reaches out to Loki, and Loki reaches out to her, but it's revealed that the moment isn't real. (Sorry, that scene is a tad confusing in the transcription) 

Jane and Darcy are next seen in a police station picking up Selvig. As they leave the station they see birds in the sky disappear, and then suddenly the birds come up from their feet. Dr. Selvig says, "This world is crazier than he I am." 

Captives from the village scene and Loki, are all shown locked in their cells. One of the village captives pulls a hammer from his belly. He breaks out of his cell and releases the other prisoners. In doing so he kills many of the Asgardian soldiers. When the creature passes by Loki's cell, their eyes meet, and Loki tells him, "You'd better go the left stairs." 

Next, Dark Elves' aircraft is seen invading Asgard. The Asgardians respond with anti-aircraft weapons. The leader of the Dark Elves of Svartalfheim, Malekith the Accursed, busts through the Asgardian palace. 

Cut to Thor, the Warrior Three and Heimdall, having a discussion, trying to figure out how to get Jane out of Asgard. They know that Loki knows the secret passage, so Thor goes to Loki for help. Something happens in which Sif saves Jane, and then we see Jane slap Loki when she first meets him. Heimdall creates a diversion. Guards spot them, so Sif decides to stick around to fend them off. Sif gives Loki a warning, telling him not to betray them. They then come to Malekith's aircraft, where Volstagg sticks around to block the oncoming soldiers. Volstagg also warns Loki not to betray them."

Loki, Thor and Jane hop on the aircraft, and then Jane passes out. Something teasing takes place between Thor and Loki, and then when they arrive at some destination all three hop out of aircraft and into some other vehicle. This vehicle is used to take them through the secret passage. 

When they arrive at the Dark World, with hopes of negotiating a peace, Loki asks Thor to loosen his handcuffs. Thor does so, and Loki quickly draws out a knife and stabs Thor. The God of Thunder rolls down a hillside. Loki follows after him and kicks Thor in the head. Thor calls for his hammer, Mjölnir, as he goes to reach for it Loki cuts off his right hand (bone is plainly visible).