I answered my own question by using the search builder at [search.msn.com...]
What one needs to do is preceed the URL by "link:". So to find backlinks to http://www.example.com/ I searched for link:http://www.example.com/
I was astonished at the result. Yahoo and MSN didn't agree by a wide margin, but in general they showed about twenty times as many links to several of my pages as google does.
I wish I knew to do this several months ago, because I've been working to promote my site for several months. I've seen increases in traffic, and just had my first increase in pagerank just last week. But google hasn't reported any increase in backlinks the whole time, and in fact the number of backlinks got smaller for one of my pages, according to google.
Here's a question for you: for a PR 5 or PR 6 page, how long do you typically have to wait for MSN or Yahoo to report significant increases in backlinks? Is it as long as a google update period? My site gets crawled frequently by all the spiders (a fact for which I realize I should be grateful, and am), but of course what I need is for the sites that post new links to me to get crawled.
One thing I do know though is that a lot of those links are of low value. I often find scraper sites in my referring page logs. Each of my pages that ranks in the top ten for any kind of competitive keyword will get a lot of links from scraper sites that mostly are a snapshot of some search results - along with some adsense of course.
What I'd like - maybe you can recommend something - is a tool that can slurp up the URLs of a bunch of referring pages and tell me all their pageranks. The point is to try to calculate my likely new pagerank myself.