Peter Van Dijck has collected a bunch of good presentations about developing scalable websites to his blog. Good stuff there, Cal Henderson in particular is always worth a read. I really should sometime collect some of our own learnings about the topic, in particular regarding how scaling challenges change when you're dealing with lots of interactive users with real-time messaging. It certainly has entirely different bottlenecks compared to a typical web site - but the profile of a typical web site is migrating in that direction, too.