Search is working again
Just wanted to let everyone know that we've got search indexing working again. Near as I can tell, it looks like the problem was due to a couple of corrupt nodes (out of about 60,000) causing the search index function to throw an exception, and then stop dead. I've patched the Drupal instance it so if it ever runs into this issue again, it'll log a warning and keep indexing.
At any rate, the short version is that everything should be indexed now, so search should be useful again. We'll keep an eye on it and make sure it continues to work properly.