I have written for the Open Group's Secure Enterprise 2.0 group a white paper about Web 2.0 security threats.
Discussing Web 2.0 security is difficult as Web 2.0 itself is somewhat blurry term. Most discussion on Web 2.0 security tends to be on the technical level, however Web 2.0 is not a technology, but rather a concept. When researching the issue, I was surprised to find out that Tim Barnes-Lee's, the Web conceiver views the original web as the 1st Web 2.0 implementation. After considering this, I tend to agree. Wikis are for sure a Web 2.0 application and they exist since 1994.