3/23 – Update – Please see this post for a proper workaround
If PowerPivot is stuck on the ASSPIInstallFarmAction installation step, see if STSADM.exe is currently running. Also check C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Log\<date>\Detail.txt. If the last line is:
<date_time> AS: Running Script: & “C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN\psconfig.exe” -cmd services -install
Manually deploy powerpivotfarm.asp globally and powerpivotwebapp.wsp to the Central Administration site. Terminate STSADM from Task Manager. The PowerPivot installation will see that the solutions have already been deployed and will continue with the installation.. PowerPivot should finish successfully.