Howtoforge posted a guide about setting up Virtual Hosting with Proftpd and MySQL
This document describes how to install a Proftpd server that uses virtual users from a MySQL database instead of real system users. This is much more performant and allows to have thousands of ftp users on a single machine. In addition to that I will show the use of quota with this setup.Virtual Hosting With Proftpd And MySQL (Incl. Quota) On Fedora 9