The short version is that through a mix of automatic means and 'standard practice' two routers were assigned the same OSPF ID. That alone would have been problematic, but the fact that these two routers were direct neighbors of the router/switch that joined the "new" network to the "old" really caused things to go SPANG!
With the clarity that only a few days off can bring, the problem came to me as I pulled into the parking lot this morning. I checked as soon as my PC booted and sure'nuff the switch/router at Central Library and one of the other core routers had the same OSPF ID. The one at the library was supposed to have the ID, so I changed the other router. An hour ago I took out the Friday emergency workaround and restored things to the way we had intended.
With the ID conflict removed, everything looks both hunky and dory. All of the switches/routers I've checked agree on the number of routes in the network, and no one is constantly running their route-table generation process (SPF).
I am, I think, now in the mood to celebrate completing the Great Network Upgrade That Ate 2005. Yeah.