Senator Richard Durbin (D-IL) has sent letters to more than two dozen of the United States’ leading technology and Internet companies, asking each of them “what, if any, measures it will implement to ensure that its products and services do not facilitate human rights abuses by the Chinese government.” And Durbin’s inquiry carries some weight: he chairs the Judiciary Subcommittee on Human Rights and the Law, and is one of the Senate’s most senior members. Companies queried by Durbin include Google, Facebook, Skype, eBay, Microsoft, and Apple.

The letter is a followup to a letter Durbin sent to a smaller set of companies last year, urging companies to adopt a voluntary code of conduct for companies operating in countries that restrict access to the INternet. Yahoo, Google, and Microsoft supported the proposed code of conduct. However, to date only Google has put its foot down, recently announcing it will no longer cooperate with Chinese government censorship on its Google.cn Chinese-language search engine and threatening to cease operations in China altogether. So far, no other U.S. technology companies have joined Google in taking a stance against Chinese Internet censorship.

“I commend Google for coming to the conclusion that cooperating with the ‘Great Firewall’ of China is inconsistent with their human rights responsibilities,” Durbin said in a statement. “Google sets a strong example in standing up to the Chinese government’s continued failure to respect the fundamental human rights of free expression and privacy. I look forward to learning more about whether other American companies are willing to follow Google’s lead.”

And Durbin might well learn something: he’s scheduled a hearing next month on global Internet freedoms next month; the event will feature testimony from Google and other companies regarding their business practices in companies that restrict Internet access and censor online content.

Durbin’s letters of inquiry follow on a recent major policy speech by Secretary of State Hillary Clinton that for the first time articulated Internet freedoms as a major plank in the United States’ foreign policy platform. China has characterized U.S. and Western calls for Internet freedom as a form of cultural imperialism.

Companies receigin Durbin’s query that weren’t included in Durbin’s earlier letter include some surprises like Amazon, IBM, Oracle, and RIM. Companies that didn’t bother responding to Durbin’s previous query include Acer, Toshiba, and…Twitter.