Here is a short post from Cyrus at IranAffairs.com
One has to ask what is meant when it is said that Egypt could become “like Iran”. If by that we mean an Islamic Republic, it is an irrelevant question. The US has no problem with one Islamic Republic (of Pakistan) and is trying to stablize another one (of Afghanistan) and in fact while Iraq is not similarly formally named an Islamic Republic, its constitution does define it as an Islamic federalist republic nonetheless. Leaving formal names aside, the US also gets along fine with repressive theocratic governments (such as Saudi Arabia.) So, all this handwringing isn't really about whether Egypt will end up an Islamic republic ruled by religious zealots.There is little that I could add to this.
However if the definition of being "like Iran" means becoming an independent country ruled by a indigenious government that is no longer a puppet regime and can no longer automatically be assumed to play along with US-Israeli interests and goals in the Mideast, then indeed Egypt is likely to become “like Iran” and I think THIS interpretation is what the Israelis and the US are really worrying about, not whether Egypt will become an Islamic Republic.
Iran and the Muslim Brotherhood are the obvious focus of anxiety by Western observers on the Egyptian situation. The people of Egypt have every right to elect embodiments of either ideology and either would be an improvement, in democratic terms, over the current dictatorship.