The question of weather or not to attack iran is "Just how sane are their leaders, and who's really in power?". When it comes to ruling iran, there is the goverment and then there's relgion. The top relgious leader has veto (i think thats the right term) powers if it wants to over every decision made by the goverment. So if they get a nuke, who actually has the big red button? and how willing are they to use it.
From what i have read, both are pritty nuts. I would not be suprised if they manage to assemble enough nukes to litterly whipe israel off the map and launched a sucidal attack using long range missiles (and it would be sucidal). Or they could give nukes to terrorists so it's harder to trace it back to them. Although israel is in the most danger, the iranian goverment chanting "Death To America" whith the unvaling of their nuclear program doesn't sound good.
This is not another "IRAQ HAS WMD's....... sorry but we can't tell you how we know this". If they can supply nuclear reactors with the nuclear fule to run them, then they can easily make fule for nukes, and it's not much of a stretch to build a nuke.
I would believe they don't intend to make nukes IF they completely opened up their nuclear program for the world to see and let the inspectors look wherever they want, and take active steps to show they don't intend to make a nuke. They have not done this. I belive the EU deal also offered to supply all the fule for their nuclear reactors if iran didn't take steps to make the fule themselves, Iran rejected this offer.
So the question should be "How do we stop them". The best solution would be to cut off all trade, i mean ALL if they don't stop. Currently russia is building their nuclear power plants and is supplying them with "Defencive" weapons such as new and effective SAM's. China is buying raw materials such as oil form iran to help fule their econemeny. I doubt both will cut off trade with iran any time soon. So that plan is out the window.
Then their's the military option. Right now a lot of coutnries are P/Oed with the US and will not help in a miltary action. I doubt the Britan will even help, so that leaves israel (proably the most effective military for it's size, against armies), which WILL attack iran if they don't stop with or without the US. Israel has barly the ability to take out all the nuclear related targets, and the US ground forces are stretched to thin and after Iraq i doubt they will attempt basicly the same thing again. So that means a MASSIVE air campain with special forces to take out specific targets. Problem with a air capmain is that no one knows where all the nuclear ficalities are. Then there are the USSR desgin bunkers built in case of a nuclear war, bunker busters may not be able to get through (i'm taking over a dozen attacks on one bunker), and SF's may not be able to get in. So the Iranians have their program set back 5 years max with a renewed effeort to get a nuke.
The losses to the america/israel depends on how well their military is trained. Iran does have the ability to defend key locations agaisnt air attack from the US, but that is only if they are smart (look at what happened in Iraq), which i have my doubts.
Then how will iran re-act. Most likely they will attempt to shut down oil flow out of the middel east by attacking tankers, oil lines, wells and infrastructure. It would require a LARGE effort to protect the tankers from all forms of attacks, and iran could send speical units into nearby countires to sabotage oil infrastructure. This would result in massivly high oil prices which would damage the american econemey and ripple effect proably through out the world.
So basicly, it doesn't look good.