They aren't the majority but they do pose a serious threat (as do the antisemitic Lebanese "Christians" I went to high school with; during Gulf War I they kept hoping that Saddam or someone would nuke Israel).
Here is a report (take it as you will) about some speakers recruiting on college campus:
Soda, pizza and the destruction of America: Klein reveals secret visit to Muslim Student Association
There are those in America and around the world who see an attack on a muslim nation (fundamentalist Islamic nation or not) as an attack on Islam. The attack on the US on 9/11 was a sign of encouragement to those who had waited silently in America. A previous attempt on the WTC had been made and a sniper shot people at the CIA headquarters prior to 2001. Some other attacks may have been made but misinterpreted but largely most attacks on America were on American interests abroad (embassies, ships, etc.). We may be in line for more severe attacks this time around. After 9.01.2001 there has been an increase in "lone nut" activity in the US.