The thing is that this script will be going out to
machines that I don't have control over, so I would
want the script files to be double-clickable as
delivered, and not require any changing of the
default handling program.
I don't think this would normally be an issue. But since it happened to you I understand your concern. Maybe distributing a .command file is not the best solution for your environment.
Are .command files always associated with the
Terminal by default?
As far as I know, yes, .command files are associated with Terminal by default. I'm not sure how your's may have gotten changed. I tried selecting a .command file in Finder and then both "File -> Open with... -> TextEdit" and <Control>-Click -> Open with... TextEdit. In both instances the file opened with TextEdit but it did
not change the association to Terminal. Double-clicking the file after editing in TextEdit still launched Terminal.
On the other hand if you did a Get Info and changed the "Open with:" pop-up then it will change the association for future accesses to the file.
Steve