Last year, the al Qaeda Internet media operation (mostly in Iraq, but in other countries as well) was taken apart. This was an aftereffect of the defeat of al Qaeda in Iraq, where many of their key Internet people were operating. These terrorist geeks were either captured or killed. Some escaped, and joined the many Islamic terrorist Internet specialists who don't get their hands dirty. These guys are pretty smart, and have useful Internet skills. They quickly rebuilt the al Qaeda network, maintaining 15-20 sites that are designed to be difficult to shut down. These sites frequently shift their IP address, and the servers they operate from. This effort employs the same tools and techniques that Internet criminals use to stay ahead of the police.
The al Qaeda geeks need these evasion techniques not to avoid the police, but to protect themselves from continuing efforts by anti-terrorist hackers (individuals and groups) to shut down pro-terrorist web sites. Sometimes these vigilantes are exceedingly effective, and lately they may have shut down all but a few of sites used as distribution points for important new terrorist videos. The dozen or so main al Qaeda sites are needed to provide distribution for terrorist propaganda (especially videos) and recruiting materials (which the videos are also a big part of) are under constant attack. It's a war you don't hear much about, but it's pretty fierce and has been going on for years.
That brings up another point. Islamic terrorists are often described as having "thousands (usually 5,000+) of web sites." But nearly all of these are basically fan sites. There are only a dozen or so sites that actually conduct the business of terrorism. While these sites are under heavy attack by anti-terrorism hackers, they are also closely watched by many intelligence agencies, and a few private anti-terrorism organizations.
Islamic terrorists have been reaching out to their supporters on the Internet, openly asking for ideas and information. This is a dangerous thing for terrorism fans to participate in. If the local police catch someone sending suggestions or information to terrorist groups, it can get you arrested and jailed. Apparently it does put a lot of people on the police radar, and eventually leads to arrests.
The intelligence agencies prefer that the Islamic terror sites stay online, so their users can be watched and identified. But the anti-terror vigilante hackers want to fight back, and no one has the heart, or the means, to stop them. The intel people know that Osama Bin Laden believed, from the beginning (in the 1990s), that electronic communications would be an essential element in keeping the jihad ("struggle" in Arabic) going. Because of this, Arab speaking (or literate) intel operatives have long participated (as users) on these web sites for over a decade, collecting much valuable information in the process.