Internet monitor Renesys explains:
Starting at 10:26 UTC [today] (12:26pm in Damascus), Syria's international Internet connectivity shut down. In the global routing table, all 84 of Syria's IP address blocks have become unreachable, effectively removing the country from the Internet.
Arik Hesseldahl notes this could be an ominous sign:
Renesys is still investigating what’s going on, but, as we’ve seen in other countries, cutting off the Internet is usually meant to try and control the flow of information to the world. It’s also a pretty sure sign that the regime of Bashar al-Assad is either getting nervous about how it is being perceived in the world, or that it is planning something unspeakably harsh in the coming days and wants as little information emerging from that country as possible.
Max Fisher wonders why it didn't happen sooner:
[Syria's] uprising long ago exceeded Egypt’s and Libya’s in severity by the time those countries had instituted their own blackouts. One possible explanation is that Syria has been far more assertive online, using it as a tool for tracking dissidents and rebels, and sometimes even tricking them into handing the government personal data using phishing scams. President Bashar al-Assad has a background in computers, unlike the much older Hosni Mubarak and Moammar Gaddafi, and once even directly mentioned his “electronic army.” Assad’s regime may have seen opportunity as well as risk on the Web, where perhaps the Egyptian and Libyan authorities saw primarily a tool of the uprising. Or, perhaps the Syrian simply feared the economic consequences of an Internet blackout, or lacked the means to conduct it.
Heavy fighting in Damascus has now closed the city's international airport as well, while earlier this week rebels reportedly used surface-to-air missiles to down two of the regime's military aircraft – a potentially significant development in capability for the rebels. The rebels have also been using children in combat roles, according to a new report from Human Rights Watch.