According to PostgreSQL.org TODO:
CLUSTER
* -Make CLUSTER preserve recently-dead tuples per MVCC
requirements
* Automatically maintain clustering on a table
This might require some background daemon to maintain clustering
during periods of low usage. It might also require tables to be only
partially filled for easier reorganization. Another idea would be to
create a merged heap/index data file so an index lookup would
automatically access the heap data too. A third idea would be to store
heap rows in hashed groups, perhaps using a user-supplied hash
function. http://archives.postgresql.org/pgsql-performance/2004-08/msg00349.php
* %Add default clustering to system tables
To do this, determine the ideal cluster index for each system
table and set the cluster setting during initdb.
* %Add VERBOSE option to report tables as they are processed, like
VACUUM VERBOSE
* -Add more logical syntax CLUSTER table USING index; support
current syntax for backward compatibility
The question is when these todo items are going to get addressed.