What I'd like to see is how much cache hit they add on each level of cache. I remember some presentation of the facebook images stack where less than 10% (if I remember well) of the requests actually hit the disks; it would be interesting to see the patterns for the whole SE galaxy.
As someone mentioned above, SQL Server hits memory first, and as they have 384GB of memory, most of the requests would sit there. That's just on the DB server. In the linked article it all authenticated requests hit the DB, with anon users getting a cached copy.
