Two interesting presentations from this years' MySQL Conference
My favorite excerpt:
Accept the observer effect
* measurement is a necessity
* it's not optional
- Wikipedia: Site Internals, Configuration and Code Examples, and Management Issues - Domas Mituzas (MySQL and Wikimedia) talks about the internals of scaling a large site like Wikipedia (pdf). There is a good section on caching with memcached, although it applies to caching in general.