According to a study conducted by Jonathan Zittrain and Benjamin Edelman from the Berkman Center for Internet and Society, Saudi Arabia uses a program called SmartFilter (whose home is San Jose, CA) to block citizens' access to a range of web sites.
The Saudi government blocks not only porn and drug related sites, however, but sites operated by human rights watch groups, like Amnesty International and the Saudi Institute.
Out of over 60,000 requests, the program blocked 2,038 pages.
The report is here.
World Tribune's coverage (the headline reads "Harvard study cites Saudis as leading Internet censor"), is here.
(Original post modified in response to Ben Edelman's post; see below).