http://technet.microsoft.com/en-us/library/cc262350.aspx#section2 Plan authentication methods (SharePoint 2010) that was just (Aug 5th) "entirely updated and revised" states (I have bolded the part I question):
"For new implementations of SharePoint Server 2010, use claims-based authentication. With this option, all supported authentication types are available for Web applications. There is
no practical reason to select classic-mode authentication for new deployments, even if your environment includes only Windows accounts. Windows authentication is implemented the same way regardless of the mode that is selected. There are no
additional steps to implement Windows authentication when you use the claims-based authentication mode."
But PowerPivot requires classic mode --
http://technet.microsoft.com/en-us/library/ee210621.aspx says "PowerPivot for SharePoint is supported for SharePoint web applications that are configured to use Windows authentication." And then of course there's Dave Wickert's post
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