I recently put together an email for some colleagues with a few links to various resources that were related to some questions about scalability vs. performance that I received. These links are good resources for any developer or systems administrator interested in scalability. I thought this might also be useful to others facing growing web applications.
Distributed Caching with Memcached - This is a good foundation article on the use and implementation of memcached.
Good memcached FAQ - Very nice FAQ for memcached. Touches on some items that are a little harder to find elsewhere.
memcached at Facebook - A little article about how memached is used at Facebook. A little dated but useful.
LiveJournal Architecture Links - Studying LiveJournal's architecture is worth your time. They made memcached.
A broader view on Scalability with some good information
XDebug and KCache Grind - Learn to profile your applications. Profile often and early to avoid problems later.
APC Opcode Cache - No well dressed PHP application should be without a good opcode cache save resources.