Microsoft has released a new sample database: Contoso. I’ll check it out this weekend and make my new screencast build on this new datasource!
It’s supposed to have millions of rows, great stuff for PowerPivot demo’s.
You can download it here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=868662dc-187a-4a85-b611-b7df7dc909fc
Update: when you have trouble unzipping the cube backup use winrar, that will do the trick.