In practical terms this means people in Cuba, North Korea, the Sudan, Syria and Iran get "403 forbidden" messages when they try to access sourceforge.org addresses. (Here's how the Armenian Private School in Toronto, Canada displays 403 errors.)
A more important question may be how far is the State Department willing to go in order to enforce this restriction and how far is the open source movement willing to go to fight it?
That's because Sourceforge is no longer the only open source repository. Microsoft has a big one. Google has a big one. Many open source projects now run their own forges. Will the U.S. government now censor Google while it ostensibly fights alongside it against Chinese censorship?
Also, neither open source nor the Internet are entirely American any longer. Just as Iranian dissidents can use anonymizers to hide their tweets from the Mullahs, so Iranian hackers working for the mullahs can use the same technology to bypass any block.
Sites like ArabCrunch, or open source advocates in places like India, now have an opportunity to mirror Sourceforge content in the name of Internet liberty. And how delicious would it be for China to allow content mirrors and enable access to those whom the U.S. Internet authorities are "oppressing"?
(NOTE: Some readers have been offended by the use of the word "oppressing" to refer to countries our government does not like. It was used in the context of how Arab or Chinese readers might react to the move. My apologies to all who might be offended.)
The law of unintended consequences is about to come down hard.