OK, it seems that when I disabled Turck support in LocalSettings.php, suddenly all my navigation links worked like magic. But I've got no caching at all now, and I'm a bit concerned about that.
After inserting a few dozen log statements, I was able to isolate where (but not why) the problem occurs. Specifically, it's in ObjectCache.php, in the BagOTurckStuff class (nice confidence-inspiring name, BTW). The function mmcache_put() was claiming to succeed with a return value of 1, but for some reason the value didn't seem to be there when mmcache_get() tried to access it.
I'm worried about going forward without some sort of caching solution. Can anybody give me pointers on why my Turck isn't turcking the way it should? I have no experience using or debugging it.
Thanks, Carlton