The latest proftpd package for Powertools 6.0 is missing /etc/pam.d/ftp. Although this file might be in other packages it needs to be in proftpd as well. Without this file an install of proftpd will not work, no one will be authenticated because of the missing PAM configuration.
There is an update to the proftpd package, it isn't in the correct place (right now anyway), but you can get the errata at: http://www.redhat.com/corp/support/errata/RHSA1999034_O1.html The update was erroniously placed under the distribution errata, so it may change to the Powertools errata by the time you get this (so look in the Powertools errata if you can't find it). I tested this package on my production machine and it appears to work, and the /etc/pam.d/ftp file is present (and belongs to proftpd). Tim