As of this writing, users of Excel 2013 can now get access to PowerPivot only if they have the standalone retail version of Excel (not the Office Suite) or a ProPlus (not just Professional) edition of the Office Suite. Believe it or not, this is actually an improvement – previously no retail versions included it.
Note: if you are an Office 365 user and you require the new BI tools, be very careful as to which subscription level you sign up to – make sure you ask! (I can’t fathom which versions include what features, but then I’m just an Excel guy)
Update: it appears that for standalone Excel 2013, the PowerPivot add-in will be streamed as an automatic update from September 10, 2013.