There is some problems with repository in title. Yast/zypper can see only main package proftpd-1.3.3d-2.1.i586.rpm and not the others, take a look:
proftpd-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 2.1M Details
proftpd-devel-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 63K Details
proftpd-doc-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 304K Details
proftpd-ldap-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 25K Details
proftpd-mysql-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 17K Details
proftpd-pgsql-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 16K Details
proftpd-radius-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 25K Details
proftpd-sqlite-1.3.3d-2.1.i586.rpm 31-Jan-2011 00:20 17K Details
I’ve tried other mirrors with same effect.
So if one have these additional packages with previous version then is UNABLE to upgrade via Yast/zypper.
In my case i’ve had to delete only doc package then upgraded successfully.