I've been thinking and experimenting a lot with MySQL 5, and PostgreSQL 8.3, trying to figure out which database server would be most optimal for scaling larger web applications.
I've brought some of my MySQL optimization tips into a small e-book styled guide, something I hope will help my fellow developers understand what they are getting into before choosing a production environment with MySQL 4, or MySQL 5 in contrast to PostgreSQL.
If you have any other tips for scaling with MySQL, I'll make sure I add them to my little e-book and release and updated PDF file :)
Scaling MySQL (PDF)