After much research, experimentation, testing and tweaking I'm happy to announce that I have completed the first part of my CentOS 5 server setup howto series!
As of today, you'll notice a new CentOS 5 Howtos link on the where I have listed the first two parts of the howto series, the getting started howto which will help you setup a basic system environment and more importantly, the mail server howto which documents how to setup a secure mail server offering POP3/IMAP/SMTP with virtual users stored in a MySQL database.
I'm very happy with this setup because it uses virtual users that cam be mapped to system users and also keeps the software set relatively small; Dovecot is used for SASL authentication (both for POP3/IMAP and SMTP) and for postfix's local delivery agent, so with only 2 servers we've got it all covered (of course technically it's 3 servers with an extra transport if you take amavisd and response-lmtpd into account).
The virtual user database is currently only used in this tutorial for the mail server, but I have plans to introduce (with an upgrade path) a new database structure that will unify several authentication data pools and make managing clients for a shared hosting server easier... But I'll talk more about that later once I've finished posting my other guides. I plan on adding ones for other services such as DNS & Web, although I cannot promise when those will be finished. The mail server tutorial alone is 16 printed pages (!) so it does take me quite some time to ensure that the tutorial is well documented and that the configurations listed work properly.
I still have to add some notes here and there about the implementation, but the core material is there. Enjoy!