At least one BBC reporter on the ground in Egypt insists that the Islamic Brotherhood is not a significant factor in the protests -- they are just a canard offered by Mubarak consistently for 30 years to scare the Americans into blindly supporting his dictatorial regime.
So the 85 million people of Egypt-- no democracy for you! The U.S. has several important interests in the Middle East-- your freedom and prosperity don't figure into any of them.
No doubt the Obama Administration is giving considerable thought to Iran these days. The U.S. backed the Shah's despotic regime for 25 years. When protest rallies spun out of control in 1979, the Carter Administration appears to have backed away from their proxy. There was short struggle for a secular, constitutional government, but when Ayatollah Khomeini was allowed to return, the radical Islamacists succeeded in wresting control of the government away from the moderates.
It could be argued that the lesson to be learned is that repression by the Shah would have been better. But you could make an equally cogent argument that the extremists were able to take control because long-term unconditional support for the Shah had weakened the moderates.
It is possible that if Mubarak leaves, the Islamic brotherhood may succeed in eventually seizing power. It wouldn't be the end of the world. Iran today is really not the caricature that the U.S. likes to present to the public, and Afghanistan and Iraq are not exactly sterling models of the kind of alternative society the U.S. has in mind. But it also may be the result of the U.S. supporting Mubarak for 30 years without pressuring him to democratize gradually, to allow the establishment of a moderate opposition with real power, and to cultivate the institutions of society that moderate political power, like labour unions, universities, and regional governments.