Unfortunately, I have NOT gotten this to work. Only on a web application that has "Classic Authentication" set has it worked. If you setup a web application to have claims based auth with both a "Forms" and a "Windows" login capability, the PowerPivot code will fail in its login method:
<nativehr>0x80070005</nativehr><nativestack>OWSSVR.DLL: (unresolved symbol, module offset=00000000000BB1EC) at 0x000007FEEFEFB1EC mscorwks.dll: (unresolved symbol, module offset=00000000002CF777) at 0x000007FEF62BF777 Microsoft.SharePoint.Library.ni.dll: (unresolved symbol, module offset=00000000000E7BAA) at 0x000007FEF23A7BAA Microsoft.SharePoint.ni.dll: (unresolved symbol, module offset=0000000001A5D823) at 0x000007FEEC87D823 Microsoft.SharePoint.ni.dll: (unresolved symbol, module offset=0000000001AD201F) at 0x000007FEEC8F201F </nativestack>Access denied.
Only way to get this to work is to reset the web application to "Classic Authentication" and then the code works. This is with the latest cumulative updates applied.
Another blog post by Denny mentions a bit more into this, but the code should work as it is calling the object model to simply update the workbook in the list on the site :(
Chris