George,

I think I should have better explained this point, sorry.

The prerequisites to use only the PowerPivot for Excel are:

* Microsoft Office Professional Plus 2010 Beta

* PowerPivot for Excel (32-bit or 64-bit)

* Windows XP with SP3, Windows Vista with SP1 or Windows 7

* .NET Framework 3.5 SP1 (Windows 7 already have this one).

But if you want to use PowerPivot for SharePoint then you'll need the SQL Server 2008 R2. I'm not a SharePoint especialist but in the oficial website we wave this as prerequisite (http://www.powerpivot.com/download.aspx), I installed SharePoint 2010 beta in a SQL Server 2008 R2 instance and went fine.

I will ask to some friends of mine that are SharePoint especialist and bring you an confirmation, ok?

Let me know if you have more questions!

You can also take a look at the http://www.powerpivotfaq.com 🙂