PowerPivot doesn't install in SharePoint mode on an Active/Passive Cluster
We just got off the phone with Microsoft about this very same question this morning.  According to MS, there is an internal document from the product team stating that this is an unsupported scenario.  Which really stinks because we wanted to leverage our existing cluster licensing for SSAS rather than paying for an additional stand alone instance.  At any rate, hope this helps.  I have requested additional guidance if this is going to be tested and supported in a future CU or SP or if MS is going to remove the capability of allowing someone to install a clustered instance of SSAS in SharePoint Mode through the installer.