The wire services are reporting comments from Iran's Intelligence Minister that they will shoot down American UAVs (spy drones as the wire reports like to call them). Since they have not attempted, or maybe not been successful, at this in the past, this seems a fairly idle threat. Additionally, other than the random type of anti-aircraft fire that Saddam's regime would throw up into the night sky, the Iranians would require more sophisticated missile and radar systems to shoot down the UAVs.
This would require the Iranians to turn on their radars in the first place, which, based on the WaPo article from Monday, appears unlikely. They do not want to give their position away in fear of losing the systems pre-emptively if the US launches a more advanced piloted mission to attack either Iran or the nuclear program facilities.
The UAVs, and even their discovery, can aid the US diplomatically. The same article above also points to the Israeli foreign minister in London today declaring Israel's belief that Iran will have a nuclear device in 6 months' time. The US, using the UAVs, ups the pressure on Iran, as the Europeans, Russians and Chinese all know for sure that the US position is more "pro-active" than the European group of three (UK, France, Germany) that has been getting nowhere in their discussions with Tehran. The US playing a bigger bad cop to the European bad cop may lead to progress.
However, as Dr. Kissinger points out in this article, there is little reason Iran would be willing diplomatically to give up the weapons technology and that pursuing a regime change policy may not happen in time.
Allowing Iran to go nuclear is not only a grave situation for the immediate future for the Middle East but, coupled with their religious fundamentalism and direct support for terrorism, the precedent it sets and possibility for export of weapons is an even greater danger for the US in the long run.