An alternative to using anonymous CVS for retrieving the PostgreSQL source tree is rsync, an incremental file transfer tool. A major advantage to using rsync is that it can reliably replicate the entire CVS repository on your local system, allowing fast local access to cvs operations such as log and diff. Other advantages include fast synchronization to the PostgreSQL server due to an efficient streaming transfer protocol which only sends the changes since the last update.
You can download the CVS repository using this command:
rsync -avzH --delete anoncvs.postgresql.org::pgsql-cvs cvsroot/
For full instructions, see the "rsync" section in the pgbuildfarm instructions.