The good thing about Al Qaeda is that they think big: The Cole, the embassies, the WTC, etc. They won't waste their time with small stuff like using single bombers on busses or restauraunts. The bigger the target the more planning they need and, hence, the greater chance of being caught.
Smaller splinter groups moving into the US scare me because they wouldn't think blowing up an apartment building or shooting up a shopping mall is beneath them. IMO these types are more dangerous than single nutjobs OR large organizations such as Al Qaeda.
It's my opinion that Al Qaeda has been effectively neutralized. The people capable of planning the big attacks are either dead or cowering in caves, defecating themselves whenever they hear anything that sounds like a plane, cursing the day George Bush was born and praying to their moon god for a Kerry victory in November. We're now turning the page to a more dangerous phase of the War on Terror(TM), mopping up the splinter groups and eliminating the societal causes of terror (ie, an entire region still stuck in the middle ages). I think that this phase will be the most dangerous part of the war on islamofascism with regards to attacks on the homeland.