If I am honest: I am probably the wrong one to answer this question. I am a security guy and not a terror specialist and the last thing I want to do is add just another useless blog post saying that it is extremely likely that the terrorist will soon bring down the Internet. However, there are good sources telling us that the terror networks are hiring and training hackers all over the place.
A friend of mine was doing his PhD in "terror research" (well there was a much more scientific title to it but this is the way I put it). When I talked with him about Cyberterrorism and the likelihood of the terrorist broadly attacking the Internet as a whole, he always claimed that he does not see a reason for it. Terrorists need the media in order to be "successful". Their goal is to catch a lot of attention around the globe – which they do with attacks like 9/11 and the mass media willingly is supporting them by over and over showing the same pictures (my fried, by the way, did not have a TV just avoid to being influences by such trends). If you keep this in mind, the Internet might be a much too important communications and PR instrument for terrorist groups in order to attack it.
There is an article on BBC on Monday supporting this theory: Inside the virtual war on terror