I opened up Excel 2010 today, and noticed that my PowerPivot tab was suddenly gone. A bit of searching led me to this very good post on how you can get your PowerPivot menu items back in the event this has happened to you as well.

http://denglishbi.wordpress.com/2010/06/21/where-did-my-powerpivot-excel-2010-tab-go/

Turns out closing excel before it completely loads can come back to bite you, easy enough fix though.