When installing PowerPivot in your farm, you will have to manually add the following line to your web.config SafeControls section of EVERY front end server, you will also have to add this back everytime you make a change via the Web Application settings page in Central Administration:
<SafeControl Src="~/_layouts/powerpivot/*" IncludeSubFolders="True" Safe="True" AllowRemoteDesigner="True" SafeAgainstScript="True" />
If you don't do this, then you will get this error:
“The referenced file ‘/_layouts/PowerPivot/ReportGalleryView.ascx’ is not allowed on this page."
This should have been done in the Solution install, but they missed that step. Hopefully next version of the .wsp will contain the addition
Special thanks to PowerPivotGeek.com for the helpful hint