Postgresql 9.1 PITR file system base backup - do I need to stop database?
If you have archiving of WAL files already on, there should be no need to restart the primary db server.. If WAL archiving is NOT already turned on, then you might have to restart the server to have the configuration change be active. (Looking at the documentation, I believe issuing a SELECT PG_RELOAD_CONF(); is good enough for archiving .. but the postgresql.conf file may specifically say "changing this setting requires a restart")

The only other requirement is that you issue a pg_start_backup('label') and pg_stop_backup() (neither of which require a db service interruption).

The PITR backup will be valid STARTING from the point of your backup..  (As in, you will not be able to recover to a point BEFORE you issue pg_start_backup unless you have another backup from earlier). Start the backup .. perform a file-level backup of all db files .. stop the backup .. perform a file-level backup of the pg_xlog files ... you're done (with the backup process).

To restore, you need to restore all the files you just backed up (plus any additional WAL files that were generated since you ran the backup) and create a recovery.conf file before starting the server.. I won't go through all the details, as you are obviously already looking through the documentation on it.  :)