It’s been a long time coming, but worth the wait.
No more beta, folks, the final v1 bits are now released on MSDN:
The client addon is listed under “PowerPivot” and the server is under “SQL Server 2008 R2”
Remember that you will also need the final released version of Office 2010 to install the addon, and the final version of SharePoint 2010 to install the server. (I suspect the RC version may suffice for either of those as well, but definitely NOT the Beta 2 releases from the Fall).
For non-MSDN subscribers, I also suspect that powerpivot.com will be updated shortly with public links to the client addon.