If you have recently done an upgrade to SharePoint 2010, and you are getting a popup error when trying to open an Excel document which says:
“This workbook cannot be opened because it is not stored in an Excel Services Application trusted location.”
the solution may not be very difficult at all.

When the upgrade occurred, most likely the Office Web Application Feature was also installed, but not activated. In your root site, go to Site Collection Features (Site Actions/Site Settings/Site Collection Administration), find the feature and activate it. I had done an IISRESET just before doing this, so it wouldn’t hurt to try that also, just in case this doesn’t fix it right off the bat.

All I can say is that it worked for me, in my situation :)