I’m not exactly sure when this happened, but it looks like Microsoft has moved past the PowerPivot 2008R2 and PowerPivot 2012 model and is now just offering PowerPivot on their website. Previously there were two options as I just mentioned with the 2012 version being labeled RTM.
Now that the 2013 previews of the office suite are out there it looks like Microsoft is officially pushing PowerPivot 2012 on everyone and no longer offering the download for 2010. Ultimately this is a good sign as there are a vast array of improvements over the 2010 version. Devin Knight (Twitter: @Knight_Devin | Blog: http://devinknightsql.com) chronicled many of the enhancements on his blog.
So head over to http://www.PowerPivot.com and get your hands on the latest and greatest version of PowerPivot that is available.