Off late, I’ve been working on a project where we’re using a simple key-value store instead of a relational DBMS like MySQL, and I’ve come to realize that if done properly, a lot of applications don’t require a relational database. Redis, being a simple key-value store is ridiculously quick, so try it out, and avoid MySQL when its not really required.
You should keep an eye on the Engine Yard blog
Kirk Haines has been working on a series of Key Value Store posts — he already did a basic intro, and most recently, one on Tokyo Cabinet. There are a few more in the works, and he’ll definitely be covering Redis 
http://www.engineyard.com/blog/2009/key-value-stores-for-ruby-part-2-tokyo-cabinet/