Does censorship affect measurement?

IRAN: Like China before it, Iran recently used its national firewall to block access to some outside web sites, including Persianblog and others [via Mr. Ad.]. National authorities can also block outside viewing of blogs hosted inside Iran.

How does blocking affect crawlers that measure and map the blogosphere? Are national firewalls, as used in Singapore, Iran, Saudi Arabia, and China, technically effective at isolating parts of the blogosphere from each other? How can we measure any distortions to blogging statistics?