Just announced at the today’s SharePoint Conference 2012 Keynote, Yammer will be included for free with SharePoint Online​

 

Yammer SKU’s and pricing

Beginning March 1, 2013, Yammer will be available to Microsoft Enterprise
Agreement customers.  Enterprise Agreement customers will be able to purchase
Yammer Enterprise via Microsoft volume licensing. Microsoft has also created a
set of combo SKUs for SharePoint Online (Plan 1 and Plan 2) + Yammer Enterprise.
SharePoint Online + Yammer provides customers with a world-class collaboration
platform and enterprise social capabilities.

http://sharepoint.microsoft.com/blog/Pages/BlogPost.aspx?pID=1049