Now that Office 2010 has been released, many people will start looking for resources to start learning PowerPivot.
Of course, the book I’m writing will be helpful when it will be published (September 2010), but you can also start with some online content on Microsoft sites.
First of all, this is the web site dedicated to PowerPivot: http://www.powerpivot.com/
It contains several videos and demos and it’s also possible to use a Virtual Lab without installing Office 2010 on your PC.
Then, there is the official help on TechNet Library, which also contains the DAX reference.
Moreover, there is a PowerPivot for Excel portal on TechNet, with many links to other content.
Finally, there is also a wiki on TechNet (look at the PowerPivot section here) with three entry pages: Overview, Install PowerPivot and DAX Language.
Outside of the “official” Microsoft pages, I want also suggest some other relevant URL: