If Iran wants the bomb, they are going to get it. The only thing we could do that might stop them would be an oil embargo, and since the Europeans and Chinese have already said that is not an option, there is really nothing we can do.
We could bomb their known nuclear facilities, or let the Israelis do it. But everyone who has thought seriously about this option has said that it won't work and will in the long run only make Iran more determined to get the bomb. The Atlantic staged a fascinating war game of various scenarios in 2004, which you can read about here. The bottom line of those experts was that by bombing known facilities we could slow the Iranian program by forcing them to disperse into smaller, more secret sites, but not stop them. They also highlight the many ways Iran could strike back, e.g., by using suicide bombers against US assets in Iraq and Afghanistan. The situation in Afghanistan is especially critical now, and Afghanistan is a place where the US and Iran have in fact worked together; Iran and the Taliban hate each other, because the Taliban are fanatical Sunnis who oppressed Afghanitstan's Shi'ite minority. But if we bomb Iran, that could easily change. Now there is a new assessment from the CSIS of the possibility of an Israeli strike, and it is equally sobering; James Fallows has a summary here. Key line from the study: "The more there is an Israeli threat to the survival of the regime in Iran, the more Iran will be determined to acquire nuclear weapons."
I think the Israelis are being foolish to get so upset about an Iranian bomb. Yes, some members of the Iranian leadership believe in a future apocalypse, but so do many members of the US leadership. That is quite a different thing from starting a nuclear war. Our experience with nuclear standoffs is that they tend toward stability and can deter conventional war, and that is what I think would happen between Israel and Iran. No doubt a nervous Israeli would here say, "easy for you to say, you are not threatened with nuclear annihilation," but to that I respond, yes, but I was until 1989, and I lived with it just fine.
I don't find the prospect of an Iranian bomb nearly as scare as the Pakistani bomb, which already exists, or the North Korean bomb, which sort-of exists (their test was mostly a failure). Nuclear weapons will only disappear when we find a way to put an end to wars between states. As long as there is the threat of war between states, states will develop nuclear weapons, and by making so many threats the Israelis and their US "friends" are only bringing the Iranian bomb closer.