Following previous reports that Yahoo is talking to Google about joining OpenSocial, we are now hearing that the decision has been made and that it will be announced in April. OpenSocial is Google’s social-networking platform that is a response to Facebook. Yahoo has been sitting on the sidelines of the fight between Google and Facebook for the hearts and minds of developers, and with this move it will place itself firmly in the anti-Facebook camp.

Becoming part of OpenSocial is in keeping with Yahoo’s stated commitment to open itself up more to third-party developers. (As I’ve noted before, it can always do better on the openness front). There is already a lot of support for OpenSocial, from MySpace to Bebo to Hi5. If developers are going to start creating amazing social applications for OpenSocial, Yahoo should welcome those applications on its site as well.

But given Microsoft’s takeover bid for Yahoo, which seems more likely every day to succeed, joining with Google appears to also be an attempt to further complicate any such deal. Microsoft, of course, is a big investor in Facebook and handles a large chunk of its advertising. If it ends up buying Yahoo, OpenSocial may be a deal that Microsoft finds it needs to unwind. Although, there is nothing stopping big Websites from supporting both OpenSocial and the Facebook platform (see Bebo).