Microsoft’s purchase of Yammer means that they have a real social collaboration platform.  Plus, they have included it in Office 365 SharePoint and E Plans at no additional cost for Enterprise Agreement (EA) clients.  The bottom line is that any plan with SharePoint now includes Yammer if it is licensed under an EA.  We also predict that Yammer will become available to everyone as Microsoft’s consolidates the licensing tools for Yammer with Office 365 later this year.
http://blogs.office.com/b/sharepoint/archive/2012/11/12/yammer-sku-plan-and-pricing-details-direct-from-sharepoint-conference.aspx