MSFT have just a published a new document about how to use PowerPivot Cubes\workbooks as data sources for your PPS dashboards. Worth a read as it doesn’t work quite the same way as a standard AS data source in PPS. A couple of things stood out for me:
Every table, including a Fact table, in the PowerPivot window is shown as a dimension in PerformancePoint. The columns in each table are represented as an attribute hierarchy under the respective dimension in PPS. In a similar manner, calculated columns in the PowerPivot window are displayed as attribute hierarchies for that dimension.
PerformancePoint uses the ADOMD.NET driver to connect to PowerPivot applications.
In order to use a PowerPivot model in PerformancePoint, you must install the new SQL Server 2008 R2 ADOMD.NET driver on the servers that are running the PerformancePoint service in the SharePoint Server farm. This is not required if such servers also have the server component of PowerPivot, that is the SQL Server PowerPivot for SharePoint, installed on them.