Please excuse the very long post. I thought it would be worth it because so many photographers here are concerned with image permanence, and because the information that's been shared with me here goes against a lot of the conventional wisdom.
Initial questions were raised in this thread: http://www.largeformatphotography.info/lfforum/topic/502249.html
I had brought up some issues based on an old research paper that I'd found, and on dodgy memories of some other things I'd read a long time ago. But I couldn't answer a lot of the questions being raised. Happily, someone put me in touch with Douglas W. Nishimura, who's a Senior Research Scientist with the Image Permanence Institute at Rochester Institute of Technology. He may know more about this issue than anybody.
The reader's digest version is this: selenium toning is not very effective at protecting silver prints from deterioration, unless toning is carried out to an extreme degree. Gold toning is better, working at much lower doses. Sulfide toning is better still. Most surprising of all, incomplete washing of the prints, leading to trace amounts of thiosulfate in the paper, may actually improve permanence. But there are many caveats and twists and turns. It's worth reading the full text if it doesn't make your eyes glaze over.
Read the full text of Doug's note on Toning and Permanence