After updating one of my SharePoint 2010 labs with SharePoint 2010 December 2011 CU, I noticed that I could not see the document preview and thumbnails as part of my search results. After some digging with Fiddler I noticed that the call to wacproxy.ashx was returning a weired error as follows:('ERROR:600:required parameters either missing or invalid')
This issue should be resolved in next CU, but for now if you are using Fast and Office Web Apps and you need to have the document preview and thumbnail in your search results, DO NOT INSTALL December 2011 CU.
So clearly someone is not testing SharePoint + FAST + OWA for CUs ;-(
A solution for thumbnails preview in SharePoint</a> 2010/2013:
It provides a customizable preview in SharePoint libraries both for most common documents like .doc, .xls, .pdf, etc.