Absolutely nothing. I'm just kidding, of course there is an extended ending after the credits roll that teases where the Marvel universe story is going to go next. I haven't personally seen Iron Man 2 yet, so I have no idea what is really going on in the sequel.
I want to be surprised by what happens when I go to this movie so I have not read the spoiler filled description below. I have simply copy and pasted the description given by our friends over at Latino Review. If you want read it and spoil it for yourself you can. I'm not stopping you, hell! I'm putting it out there for you! I've heard there is also bootleg video of the ending which I will not post. But if you want to see it, I'm sure you'll be able to find it.
So you've been warned that this is a Spoiler and by reading this you will ruin the very end of the movie for yourself. Last chance to turn back....
HERE IS THE SPOILER FILLED DESCRIPTION
Here's what happens:
During the movie, S.H.I.E.L.D. agent Caulson tells Tony Stark he has something he has to deal with in New Mexico. Nick Fury also mentions in the film that there are things happening in the Southwest that he can't discuss.
We see a black car drive through the desert of New Mexico. They make sure to tell us it's New Mexico by showing a closeup of the license plate which reads 8RE 2Z1 New Mexico. The car door opens and Agent Coulson walks up to a giant crater. There are trucks driving around a big hole in the crater. Agent Coulson turns around, gets on the phone and says, "Thor. We've found him." (He could also be saying, "Sir, we've found it." I couldn't exactly tell, the audio wasn't that great).
The camera then cuts to a shot of Thor's hammer stuck in the ground in the middle of the crater.
So there you go, that's your end scene for Iron Man 2. The build up to Thor and The Avengers continues!