Saudi Arabia, Egypt, and Turkey will feel pressured to respond with their own nuclear weapons. It will be interesting to see if they can buy them from Pakistan or Israel, which would probably be preferable to their building their own development programs.
Iraq may go the same way, but they may play it smart instead and negotiate a permanent basing arrangement with the US similar to Okinawa in exchange for guarantees. This is the last thing that Iran would want, so they will probably hold off testing until nearly all the troops have left.
India, Pakistan, and Afghanistan would be unaffected. The first 2 already have their own nukes, and the 3rd lacks targets of value (or anything else). al Queda and the Taliban are extremist Sunni organizations that Iran would like to see reduced to impotence, even if they provide some logistical support just to bleed the US more.
Europe, China, and the UN -- no change. It's all the fault of the US, as always.
Russia is the real question mark. While they have been playing a lot of diplomatic games, an Iran with real nuclear weapons controlled by religious fanatics has got to be one of their nightmares.
� 1998 -