PowerPivot for SharePoint existing farm installation guide (draft paper by Microsoft)
Use this document to configure a PowerPivot Server on a SharePoint farm if:
1. You already have a SharePoint 2010 farm installed and configured and you have identified the servers that you want to install PowerPivot on. Go directly to step 10 if this is the case.
2. You want to set up a multi-server SharePoint 2010 farm and then install PowerPivot on the servers of your choice. (Please note that SharePoint 2010 requires a 64-bit platform and Windows Server 2008 SP2 or later.) You will be guided through the steps to create a farm that looks like the following. It should be relatively straightforward to adapt these instructions if your farm is slightly different.