I made it work! Huuzzzah! I guess I just had to hit it like an old toaster. Here’s what I did (make sure you have time to try it, because it worked for me, and I’m not trying it any other way).
1. Send a pingback from one post in v1.5.1 blog to another post in the same blog. Confirm that pingbacks do not work.
2. Set up a brand new v1.5.0 blog (hope you have a back up).
3. Send a pingback from one post in v1.5.0 blog to another post in the same blog. Confirm that pingbacks do work.
4. Send a pingback from one post in the v1.5.0 blog to another in the v1.5.1 blog. Confirm that pingbacks do work (they should by now).
5. Send a pingback from one post in the v1.5.1 blog to another in the v1.5.0 blog. Confirm that pingbacks do work.
6. Send a pingback from one post in v1.5.1 blog to another post in the same blog. Confirm that pingbacks do work.
That’s the best I can do to describe it … and it still doesn’t make sense. But at least the pingbacks are working.
BTW, I am using this permalink structure:
/archives/%year%/%monthnum%/%day%/%postname%/