There was a fellow on one of our internal aliases who hit the following error message:
You do not have permissions to open this file on Excel Services.
Make sure that the file is in an Excel Services trusted location and that you have access to the file.
If you know that you do have access to the file and still cannot open it, you will need to make sure that the location of the file (i.e. the document library or URN it is in) is set up in Excel Services as an allowed location.
Now, Trusted Locations are an administrative feature - it's not something just any user can do - one needs to be an SSP admin to do it. That is why we cannot give the user a "Add location to trusted location list" link right there and then on the error message.
Going to the "Adminstrative Tools" and choosing the SharePoint Central Admin link will bring you to the SharePoint admin site from which you can reach the SSP settings and add the location to the list of available locations.