I have a 2008 R2 x64 server running Office 2010 32-bit. If I install the x86 version from http://www.microsoft.com/en-us/bi/powerpivot.aspx
everything installs fine for me; however, when another user logs into the server, the add-in is not checked as a start-up add-in. The user will then tick the box, close Excel, re-open, and it's still not loaded, and the check box to load at startup is empty.
As a test, I removed the COM Add-in from my profile, but was then unable to reload it (poiting at the VSTO file), getting "is not a valid Office Add-in." The only way I can get it back into my profile is to remove it and then reinstall it.... and that still doesn't change the matter for everyone else who can't load it all all.