Yahoo offered up an official statement about Google's OpenSocial APIs:
“Social networking platforms are an emerging space and we believe there is a need for the industry to collectively develop several different sets of common standards to drive growth and innovation in this area. While OpenSocial is currently just a proposal, we are very hopeful this effort will gain the broad industry support required to develop into one of the widely adopted, common sets of socialization standards the industry needs.”
In parsing this statement, it sounds like general acceptance for the concept of standards collectively developed, and appreciation for the OpenSocial "proposal" and "effort." Yahoo is "very hopeful" that OpenSocial gains "broad industry support." So it would seem that Yahoo isn't ready to get on the Google's OpenSocial train, but is hanging around the station just in case.
The larger question is how Yahoo plans to do to give its massive audience of email, instant messaging, groups, Flickr and other services social networking services.