It's been leaked. Someone posted a special ending after the credits of Iron Man 2 on youtube. the video has since disappeared but I'm sure some websites still have it up.
Here's a link to the video if you really want to see it:
If the video is removed here's a description of it from /film:
After the credits conclude, we see a black car pushing up dirt as it zooms through the desert. We are even given a close-up of the New Mexico license plate of the car so that we know it is New Mexico. The license plate reads 8RE 2Z1. I’m not sure if there is an inside joke or easter egg in those numbers and letters. Probably not.
A man exists the vehicle, and it cuts to a wide show of a huge crater in the earth.
We see that the man is… not Tony Stark — but Agent Coulson.
He looks down at the inside of the big hole, which has been revealed to contain cars and workers working on something.
Coulson picks up his cellphone and tells the person on the other end “Sir, we’ve found it.”
Cut to a shot from inside the giant crater, and we push back to reveal the hammer of Thor sticking out of the ground.
Sweet. A Thor tease. But remember how X-Men Origins: Wolverine had many different secret endings? There might be multiple ones for this movie also.
