A lot of GP users got an introduction to PowerPivot for Excel at the GPUG Summit. If you want to learn more, there is new book out that is getting great reviews. Check out PowerPivot for Excel 2010 – Give Your Data Meaning for a nice PowerPivot resource.
PowerPivot for Excel Book
Excel & OfficeMicrosoft & the GP Ecosystem
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.