I think I'm pretty late to the party on this... I'd heard that the LinkedIn alumni search was really useful but I've only recently started playing around with it. I knew you could use it to filter by Where they live, Where they work and What they do, but I hadn't scrolled right on the search panel to see that you can also filter by What they studied, What they're skilled at and How you are connected. And all this on a free account. What a great feature!