Just in case you didn’t already know, we just uploaded the PowerPivot Security Architecture technical diagram on to sqlcat.com / MSDN. To get access to the PDF, XPS, and/or VSD files, please click through to: http://sqlcat.com/whitepapers/archive/2010/08/17/powerpivot-technical-diagram-powerpivot-security-architecture.aspx
Enjoy!