On the Yahoo Manila Dev Group, David clears up most of my pinging issues:

Bottom line: Trackback is basically a cross-site commenting technology. Heck, I’d call it that “Cross-site comments”. Others have come to a similar conclusion: http://kalsey.com/blog/2003/02/simplecomments/

I’m writing a comment about something you wrote, and I want to tell you that I’ve done that. From a technologist’s perspective, that is called “pinging”, but from a user’s perspective it should be called “Notify” or something even more obvious (e.g. “Send my comments to original site [url]”).

If I want you to tell me when you comment on what I’ve written, I must enable notification, which technologists call “incoming pings” but I would suggest that users need something simpler here too (e.g. “Receive comments from other sites”).

Similarly for “URLs to ping”, I’d suggest “URLs to send my comments to.” Similarly for “Incoming pings require approval”, I’d suggest “Incoming comments require approval”

I guess I didn’t understand that Trackback wasn’t automatic, that I had to turn it on for every post or comment. Hmmm…now I have to think about why I wouldn’t want a particular post to be pinged or pingable…