It's highly likely that the next major terrorist attack in the US will come from home-grown sources. Before 9/11 the biggest terror attack was the Oklahoma City bombing, and even after 9/11 the most damaging terror attack -- the anthrax letters -- was almost certainly from a domestic source.
With the election of Barack Obama there's also a chance white supremacists will be revitalized. This story out of Maine is worrisome: http://rawstory.com/news/2008/Slain_white_supremacist_had_components_for_0309.html
While it makes good TV dramas to show foreign terror cells made up of swarthy young men in our midst, real world experience suggest that real terrorists need to be able to blend into the population. They need to be able to case targets and approach without detection. Everywhere that's been afflicted with persistent terror, from Northern Ireland, to Israel, to Britain, India, Sri Lanka, Germany, etc. the terrorists are either native to the country or part of a large minority within the country that are a part of everyday life.
The 9/11 hijackers took advantage of the cosmopolitan nature of air travel and flaws in the screening system to make their attack. The attack also relied on surprise to succeed and anything like that is unlikely to succeed again. Indeed, any attack that relies on seizing control of a transport vehicle probably won't work now because passengers and crews now fight back.
Instead successful attacks in recent years have generally relied on infiltrating suicide bombers or assassins into crowds, which is obviously easier if the assassin can blend in.
Our next terror attacker may very well be some American.