I think the kind of Islamism that is likely to arise in the Arab world is of a different variety. For the most part, it doesn't necessarily imply extremism at all. Primarily, it's just a socially conservative political movement. It's true that Islamist parties tend to be anti-US, but even this is frequently misconstrued in the United States. It has less to do with some fundamental hatred of American values, and more to do with the fact that they were the only opposition allowed for decades in stagnant autocracies. To oppose their own governments meant to oppose the United States. Had things played out a little differently, it might be the liberal parties incorporating anti-Western sentiments into their platform.
Ultimately there's no reason to fear Islamism in the Arab world, at least not yet. Even in Iran, it isn't so much Islamism that the people are so sick of (although being told how to dress by the ayatollahs certainly doesn't make them more likable.) People are sick of their government's corruption and abuses, much like they are in the Arab world. And even in the government itself, its opposition to the United States has less to do with Islamism and more to do with the fact that the US opposes Iran's strategic interests in the Middle East, and vice versa.