There is no realistic call for invasion of Iran. I don't know where you dreamed this up from but it's just not there. The only reason there is a slightest hint of people thinking about it is because no one wants to see a nuclear Iran. Yes, we'd roll right over them. I'm pretty sure they wouldn't want us there but that is irrelevant. Yes, they could do gorilla warfare, just like what is happening in Iraq and what is happening in Afghanistan. The point is that we would roll over the Iranian military establishment.
If we wanted to go a step further, the regime is already on unstable footing. It wouldn't take a whole lot for us to disrupt the regimes forces, and let the Iranian people already in the streets do the rest. Having a few division sized formations set loose in the Iranian interior to help secure the major cities would go a long way. The greatest risk is the stranglehold the regime has on the mechanisms of civilized life. We saw in Iraq what happens when the trained bureaucrats pack up and go to ground. In this case I think there is enough opposition politicians to fill the gap, so that we can avoid a long term occupation. On that same note, thats why it is so vital at home to avoid dependence on government.
Re-read my post No occupation = No guerrilla war No large formations, no taking the cities, nothing like that.