1) I'm afraid that the Nuclear genie is going to get set loose in Iran, regardless of who is in power in Iran. A change in government might provide a more serious and responsible shift in the way Iran conducts its foreign policy, but the program WILL continue. The Iranian state has too much invested in the project and it WILL make Iran one the key powers in the world. When we Americans think of American "exceptionalism" (Which I believe in), in a similar vein Iranians think of their past when as Persia, they were one of the world's premier civilizations and that they have a right and duty to once again occupy that equivalent position in the modern world
2) Obama, despite his overheated 2008 campaign rhetoric when talking about winning the war in Afghanistan, hasn't the will to follow through in that theater. What's makes anyone in their right mind think that he will do anything militarily with regards to Iran? And as far as sanctions go, it won't happen..Russia will balk at them as long as the current regime is in power and China will always balk at them because it would cause Oil to skyrocket in price and endanger their very export driven economy.
3) Israel might have the will to act militarily, but they don't have the conventional resources for the prolonged extended campaign to do it. It will come down to a "MAD" policy with the rest of the middle east. Israel's only real option will be to announce they too have nukes and will target all the Middle East if attacked by Iran... That way everyone in the region will have "skin" in the game and put pressure on Iran to behave once they get nukes.
4) Best case scenario: Iran has regime change before it gets Nukes and they and Israel join the Nuke Power club as a responsible members. Unfortunately, it will also drive other powers in the area to launch their own Nuke programs.
5) Worst case Scenario: Iran gets Nukes with the current regime still in power and then as internal dissatisfaction grows in Iran and threatens Ahmadinejad's hold, Iran lashes out against Israel to create a crisis as an excuse to stamp out internal reform.
6) Either way: Congrats Obama: It happened on your Watch.... /s off