(The following does not contain spoilers for Iron Man 2, but those being extra careful should probably skip it anyway.)
EDIT: It's also come to my attention that inconsiderate people are spoiling in the comments, so think twice before you click THAT, too.
Apparently Marvel hasn't been as johnny-on-the-spot at keeping the actual content of the "stay through the credits" tag of Iron Man 2 secret as opposed to the first one, and people should be aware that some sites are being less than diligent about spoiler warnings. (Though I'm also hearing that the movie-proper, which I've not seen yet, may actually have so much other-franchise-referencing in it's actual plot that spoiler-proofing may be impossible - the words "feature-length Avengers trailer" have been used more than once.)
For the record, as of last night there were at least 2 pirated snaps of it floating around (I'm sure the whole thing is on the youtubes by now) one of which is (supposedly) innocuous i.e. "that could be anything" and one that gives it all away.
I saw the first one before clicking away RIGHT away last night, I reccomend you don't go looking at all in case there's something in there more obvious that I missed. For what it's worth, what (I think) I saw could reasonably be one of about three things, at least two of which would be RIDICULOUSLY ballsy, game-changing stuff.
Are you excited? I'm excited.