The request would simply to have a "Subscribers:" field listed either under the "From:" or "Via:" field in the Details tab with a clickable link "Show current subscriber list".  When clicked, that list could simply pop out a bare list of attached/subscribed users in the manner of the "From:" field pop out window.  It wouldn't need to have all of the features of the "From:" field popout, a simple list of names/email addresses should be sufficient.

It would be nice for the field to only show when there are additional subscribers beyond the person who initially created the ticket.