Everything I have seen so far about the U2 incident in Iran seems to be the typical biased American Imperialisim jargon. For the life of me, I could not find anything that did not mention 'American Imperialisim' etc. It isint that that iritates me, though I am an american. I just cant find anything that dosen't sound like some baseless theory from some wingnut conspiracy theorist, and nothing that is not so obviously biass.
Likely a malfunction in the airframe or pilot error was the cause of the crash, though the second is less likely. Though to say it is impossible it was shot down is not entirely truthful. It's remote, but possible. After all, the Iranians have the same missles the Soviets did back when they shot down Power's plane. Though I imagine the U2 has had several upgrade programs to assist it. The Iranians have not said they had shot down the plane and it dosent seem to have crashed in Iran, as no airplane crashes were reported to anyone.
As far as Iran's claims they shot down some UAV's, that seems more likely. However, Iran would probably claim they shot down Santa's Sleigh if they could. So it's dificult not to take it without a grain of salt. And if they did, it is unlikely the U.S would admit to it any time soon.
And keep in mind. Internet Rumors have a tendancy for not being entirely truthful.
North Korea is unlikely, but China could probably shoot down a U2. I dont know that much about missiles, but to quote Andrew Brooks of the International Institute of Strategic Studies:
"The Soviet Union could have hit it, or today's Russia or China, but we are talking now about looking at mujahedin, at terrorists, at Taliban, at insurgents. These sort of people could not produce the capability to knock it down,"
Keep in mind, he was saying his reply in regard to the possibility of Afghani or Iraqi insurgents knocking it out of the sky. He did not mention anything about Iran nor was the question posed to him.
Aparently, U2's have crashed in South Korea as well. So a technical error is definately likely.
Here are some links for you.
(WARNING: The last post is a blog and thus inheritly opinionated. It should be taken with a grain of salt. He compares the second Iraqi war to the invasion of Nanking for Chrissake.)
EDIT: Forgot to mention. I hope this helps you out.