The order in which you install SQL Server 2008 R2, SharePoint 2010, and PowerPivot for SharePoint on a single server currently matters.  There is a glitch that occurs when you install some components of SQL Server 2008 R2 prior to installing PowerPivot.

This is the order that works for me:

1. Provision a new Windows 2008 SP2 server.
2. Add IIS role.
3. Make it a domain controller with dcpromo.
4. Install SharePoint 2010 prerequisites.
5. Install SharePoint 2010, do not check configure at the end.
6. Install PowerPivot instance (choose new server) from SQL Server 2008 R2 installation.

Now you may install any other instances of SQL Server 2008 R2.

 Here is a link that helped me identify the problem:

http://powerpivotgeek.com/2010/06/14/installation-failure-could-not-load-file-or-assembly-microsoft-analysisservices-sharepoint-integration-dll/