http://technet.microsoft.com/en-us/library/ee210621#bkmk_auth say:only Windows authentication is supported for a PowerPivot for SharePoint deployment. In addition, the web application through which sign-in occurs must be configured for classic mode.
So the default claim based authentication of SharePoint 2013 will not work?
How else can I have power view against SSAS in tabular?
Ofer Gal