If you follow this blog, you are already aware about the issue with SharePoint 2010 August Cumulative update and search in anonymous mode (result returns only pages, no Office document).
Microsoft Support has now found the origin and this is really interesting.
Indeed, the issue is not coming from SharePoint 2010 (or Fast) but from Office Web Application.
If you have SharePoint 2010 AND Office Web Application installed, search doesn’t return any Office document accessible in anonymous mode.