Regime change in Iran
The US military attack on Iran will, most certainly, be conducted with air power. The US has neither the available ground forces necessary to invade a large country like Iran, nor the appetite (given the experience of Iraq) to manage its aftermath. In contrast, airpower assets are plentiful and its employment offers a clean, seemingly low cost alternative to a ground invasion.There are two major problems with deploying airpower. The first is that Iran has both dispersed and hardened its nuclear related facilities. This situation means that in order to guarantee the destruction of some of these facilities, a nuclear weapon must be used. This is not a viable option. The use of nuclear weapons in any form is an anathema to the world and most people within the US government, despite the ability to modify these weapons to reduce their size and fallout. As a result, it is highly probable that some of Iran's facilities will survive conventional air attack.
The second problem is equally as difficult. Most of the threat posed by the Iranian nuclear program isn't contained in the facilities but in the knowledge of its engineers. This means that any destruction of facilities would only result in a short-term delay in development and a redoubled commitment by Iran to accomplish the task.
These problems indicate that the only way to truly realize a reversal in the Iranian program is regime change. Therefore the objective would be to remove the clerical regime from power -- it's likely that merely a political reshuffle would be insufficient to ensure any meaningful reduction in the threat. Additionally, this is a real test of the Bush doctrine of pre-emption. Iran has clearly supported international terrorism and will soon be in a position to supply these groups with nuclear weapons.
Of course, just as things didn't exactly work out as planned with the Iraq invasion, so optimistic gleanings like these should be judged accordingly.