I think there are more and more American extremists every day. The invention of the Internet has only increased the number of Jihadists that join the cause. They now have access to videos and other propaganda that increase their extremism. The Boston Marathon Bombings are a very recent example of two kids who were essentially American and still became Jihadists.
The lack of boundaries that are inherent to religion mean that no matter what the religion is there will always be sympathizers in almost any country. In addition rebel groups tend to recruit in areas where they believe supporters will not be suspected because it allows them a greater reach. Essentially there are and always will be sympathizers with any large religious movement all over the world.
I think you will always find people who will have sympathy for others and their situation. I feel that a lot of Americans feel bad for the current situation in Iraq and can understand why people are rebeling. Even if you feel bad for them does not mean that you are against the US
Yes, there are American sympathizers with the Iraqi rebel jihadists, because there are those, even in the United States that sympathize with the terrorists. These people do not share the same values that the vast majority of us do. Unfortunately, simply having an American nationality does not mean that you share our values and beliefs.