Great Introductory Video on Scalability from Harvard Computer Science
Professor David Malan gives a very good lecture on scalability for dynamic websites. It's not highly technical, it's an extension course, but it's a great introduction to a wide variety of topics. I really like his teaching style. He continually asks questions, prompts for input, and gives accessible explanations. Some of the topics covered: vertical scaling; horizontal scaling; PHP acceleration; load balancing: DNS, L7, sticky sessions, load balancers; caching; MySQL: replication, load balancing, partitioning, high availability.
Watch it on Academic Earth
This is one lecture in a series of 13 lectures on building dynamic websites. Students learn how to: build dynamic websites with Ajax and with Linux, Apache, MySQL, and PHP (LAMP); set up domain names with DNS; structure pages with XHTML and CSS how to program in JavaScript and PHP; configure Apacheand MySQL; design and query databases with SQL; use Ajax with both XML andJSON; build mashups.
For the lecture notes go to the OpenCourseWare site.