When can I clean up archived WAL files?
WAL Files are a record of all the transactions that have taken place .. but that's it - a delta between state before and state after, essentially.  Which means you have to have a BASE to start off of before you can apply any WAL Files.

If you clear out WAL archives without creating a new BASE, then you will be in trouble when you try to restore your database (via PITR) from them.

If you are going to delete WAL files, then you need to make sure you have a valid base to build from again afterward.

Read the documentation on WAL archiving for more information.

In terms of streaming replication, I believe that only uses files located in pg_xlog...  (files are not archived before they are sent, IIRC)


  If that's the case, and we're doing a pg_dump of our master every six hours, could I update our backup script to also clear out all archive files that exist at the start of the pg_dump (once the pg_dump is complete of course)?


Not entirely ... pg_dump creates a file that contains the information of your whole database that can be given to pg_restore...  For PITR, you actually need to create a file-level "backup" by copying your database files and WAL files (after issuing a pg_start_backup)