Harry,
The service is called LDAP_DS (directory services) which is free and totally different from LDAP_UA (user authentication).
You'll need a ldap server with the cfgs from a PC client installation...
For DB-lib & CT-lib on each client machine, you'll modify the libtcl.cfg to point to the centralized service.
For ODBC, OLE-DB there are special string properties to point to the LDAP Server.
There was a bug in the very early versions of PC client 15.x but I believe it got fixed.
For java, there is a J-connect feature to get it working.
The configuration also allows for a secondary LDAP server if the first one does not answer.
Many CRs around 2004-2005 in the 12.5.1 releases were created to get the feature working.
infocenter.sybase.com/help/topic/com.sybase.infocenter.dc35823.1570/html/uconfig/CFHGJIBH.htm
infocenter.sybase.com/help/topic/com.sybase.infocenter.dc01085.1570/doc/html/jon1242759920782.html