There problem may have been the guid, after all. I was checking for uniqueness in my new installation’s database. The problem is that Google Reader will have cached the feed from my old installation and between the two, I was definitely recycling guids. I’ve kept my old, free account exactly for this reason, so I can compare and troubleshoot weird things like this.
This post should generate a guid well past the last one either feed used. WordPress inserts the internal post ID into the guid field for the feed, so the problem is if you do anything to alter those post IDs in the database, like a fresh import from XML, you could encounter this same problem.
The fact that the imported guids had my temporary URL in them was helping to hide the issue, since the non-unique post ID combined with that temporary URL kept the freshly imported posts from colliding with the ones already posted to my old site.
Basically, from here on out, things should work. I also realize that for those who were subscribed through the site changeover, you inadvertently got clobbered by the way I handled this. I chalk this up to a lesson learned and know enough to do better preparation work should I ever have to move the site, again, to keep the feed from acting weird with Google Reader’s particular caching strategy.