@cbae: Please don't derail my question. For the record, Yahoo caches mapped index dumps, and does not pull results from Bing live. They serve the results from a database local to Yahoo. I was on a dev team with somebody at Yahoo a while back.
I think they get the results via dedicated application servers MS set up for them, then they cache them locally at central server and serve them back out to the edge servers which have them memcached for hit/miss delivery. On the web side they run free open source edge servers under the Apache project: