Rumors began spreading early this morning that phone and Internet services were failing in major Syrian cities, but several Internet traffic companies have now determined that every single Syrian IP address has effectively been blocked. Syrian activists on Twitter began to notice this morning that they were suddenly unable to get a hold of friends and relatives through the usual means, though it wasn't clear if power outages or some unknown disruption in service was to blame. However, given the fact that all Syrian internet hubs are currently unreachable and traffic has evaporated, it looks more like a deliberate effort on the part of the Syrian regime to cut off contact with the rest of the world.
Another Internet monitoring company, Renesys, says that there are a handful of small networks operating in the Syrian webspace, but they are likely located outside the country and unaffected by whatever mechanism shut down the other service providers. There has been talk online about some Syrians being able to transmit from "revolutionary media centers", but without a solid mobile connection even they may be cut off at this point. By shutting down the traffic at the source, even most attempts to route around blockages and filters would be useless.
If the regime is behind the blackout, why now? This civil war has been going on for more than 20 months, and Assad's forces have never attempted a communication blackout of this scale. In fact, the Syrian revolt has been one of the most heavily documented conflicts in history as YouTube videos and Tweets have brought powerful images and stories to rest of world with startling speed and accuracy. Without Twitter, Facebook, and the like the revolution may have never even started and the world would certainly be unaware of many of the atrocities taking place there.
In addition to the technological embargo, most outsiders have also been physically cut off from Syria as well, as airlines have canceled almost every flight in and out of the Damascus airport. Rebel fighters were reportedly descending on the area over the last day, and heavy fighting has likely caused the airlines to stay away.