The spec is now published. factoryjoe says:
Developed through the standardization of the best practices of several well established proprietary industry protocols, OAuth is similar to Google AuthSub, FlickrAuth, AOL OpenAuth, Yahoo BBAuth, Upcoming API authentication, and Amazon Web Services API authentication.
However, OAuth is non-proprietary, and does not require a specific user interface or interaction pattern. Service Providers do not have to specify how they authenticate Users, making the protocol ideally suited for cases where authentication credentials are unavailable to the websites, such as with OpenID. OAuth is designed to complement , rather than replace, authentication protocols such as OpenID.
