They are not a direct threat to the US mainland and never will be, but you are forgetting that this war is based on terrifying the enemy. That's why the decapitations, the rape of women, etc.
Terror attacks have been happening repeatedly in the US with the DC snipers, Fort Hood murders and the Boston Marathon killings getting the most publicity. There have also been several unsuccessful attempts. This is how the jihadists will attack. One person will kill one, three or thirty, it doesn't really matter. We have seen the same thing happening in the UK as well. And each time this happens, if the media reports it, we call them temporarily insane, or workplace violence, etc. But of course there will be more of this and, I believe, it will increase.
You have to think like a terrorist in order to understand how they can defeat their enemies. They certainly aren't going to try it through traditional means.
Here's a list of some of the killings in the US since 9/11, though I don't think honor killings should be included. Nonetheless you get the idea.
List of Islamic Terror Attacks in America
