For Yahoo, this is yet another step in its effort to be the starting point on the Web for its users. The nice thing about the OpenSocial apps is that users don’t have to leave Yahoo to engage with them. So it is really Yahoo’s way of remaining a destination site and keeping its users within its walls, even if they are using non-Yahoo services.
After all, users now don’t need to leave the interface to e.g. crop photos, transfer money, share large attachments with others, and so on, and Yahoo is not restricting itself to using or building proprietary tools.
I think it is important for librarians to be aware of the latest changes with Google, Microsoft and Yahoo, since so many of our customers use them on a daily basis.