This is a great mail setup, and I recommend it highly to anyone looking for a good, robust, easy to manage solution for providing different clients (i.e. domains) with mail.  I have used Fedora Core 5 version, and now the CentOS 5 versions.  I couldnt be happier, except....

In it's current form, if one user authenticates and sends an email via this server to another user on the same box (either same domain or different domain), it is likely they will be flagged as spam under one of the dynamic IP lookup rules - actually they are likely to be flagged under a number of rules, and will most likely exceed the spam level. 

The solution to this is to add another postfix setting in main.cf:

smtpd_sasl_authenticated_header = yes

This has TOTALLY solved this BIG problem for me.  Refer to http://wiki.apache.org/spamassassin/DynablockIssues form more information - about half way down the page.

Falco - I recommend you incorporate this into the actual instructions above, and any other guide you are doing with Postfix 2.3+ and SpamAssassin 3.1.4+ .  Thanks for the guides - keep them coming.