According to Jennifer Granick, executive director of the Center for Internet and Society at Stanford University Law School (quoted here), Wikipedia is unlikely to attract legal liability for libel. This is because the act, in Granick’s interpretation, protects internet services classified as "ISP’s" rather than as "publishers" from liability.
The C-Net article explains the background:
In one of the most famous cases testing a service provider’s liability, Zeran v. America Online, the 3rd U.S. Circuit Court of Appeals, located in Philadelphia, ruled unambiguously that online services like AOL, Amazon.com or Wikipedia are protected under the CDA… "By its plain language, section 230 creates a federal immunity to any cause of action that would make service providers liable for information originating with a third-party user of the service," the 3rd Circuit wrote in its 1997 opinion.
[Please note: The above post appears for information purposes only and was not written by a qualified legal professional. The Law Society can help you find a solicitor in England and Wales. Please also note the the above post is about a the opinions of a US based legal academic on a matter of US law – NOT UK law where the situation is likely to be different.]
Update: The incident on Wikipedia that inspired this blog entry has been in the news again. See http://news.com.com/2100-1038_3-5990678.html