Today I added a new server to PostgreSQL's host-based authentication file (pg_hba.conf) on a very active PostgreSQL server. Restarting the PostgreSQL service would have lead to service disruption across multiple services. Thankfully the folks at PostgreSQL are one-step ahead of me on this one. There is an option to simply reload the configuration files which, if your configuration updates are up to snuff, won't cause any service distruptions:

$ su - postgres
$ /usr/bin/pg_ctl reload

Source: http://www.heatware.net/databases/postgresql-reload-config-without-restarting/