Then, I realized that in the rebuild of the PowerBook, I had created a "testuser" account first, and then constructed my usual user (we'll call it "user") account. This meant that testuser had a User ID (UID) of 501, and my user account had a UID of 502. This, apparently, was a the root of my problem when transferring files between my PowerBook and all my other machines (all of which had a UID of 501 for my user accounts).
The solution, for me, was to use NetInfo Manager, found in /Applications -> Utilities:
- Logout as my usual user.
- Login using my "Administrator" account.
- Launch NetInfo Manager, and unlock it with my admin password.
- Select users, select "testuser" and change the value of the UID to 502.
- Select users, select my "user" account and change the UID to 501.
- Save the changes and quit.
% cd /Users
% sudo su -
% chown -R testuser testuser
% chown -R user user
After logging back in as "user," I have had no problems with network file exchanges between these various machines. It seems that keeping UIDs consistent between peers on a network may solve some annoyances!
[robg adds: I can't really test this one, so I'd welcome any comments on the subject of UID differences causing issues between machines on the same net...]

