We have a test MOSS server up and some of the users want to start
playing with Excel Services (we do have the enterprise CAL)

I have gone to Central Admin > SSP and set the Excel Services Trusted
File Location as the url of our root site of the site collection and
selected 'children trusted'

In the site collection administration pages, I went to SITE FEATURES and
made sure ENTERPRISE SITE FEATURES was activated.

However, our users still get permissions errors. Am I missing a step? Do
individual sites need to be trusted? Do users somehow have to be granted
permission (these are site admins at the moment).

They're also asking if an external network share can be 'trusted' for
MOSS. I really don't know much about excel services yet, but it's my
understanding that to use it, one would have to have the excel file
physically uploaded to their sharepoint site, correct?