Truth be told, between trips to Yahoo! including last year's Drupal conference, which they hosted, we already knew that Yahoo! is a major Drupal user. However, what is new is that they are starting to launch more and more external facing Drupal sites. Once such example is Yahoo! Research; check it out at http://research.yahoo.com.
Now if only we could get them to pay Rasmus to work on Drupal core a bit. ;-)